{"id":1167,"date":"2020-10-29T15:07:14","date_gmt":"2020-10-29T13:07:14","guid":{"rendered":"http:\/\/tecniques-metodes-recerca.recursos.uoc.edu\/7-7-medidas-descriptivas\/"},"modified":"2020-12-14T13:47:39","modified_gmt":"2020-12-14T11:47:39","slug":"7-7-mesures-descriptives","status":"publish","type":"page","link":"http:\/\/tecniques-metodes-recerca.recursos.uoc.edu\/ca\/7-7-mesures-descriptives\/","title":{"rendered":"7.7. Mesures descriptives"},"content":{"rendered":"<p>L&#8217;estad\u00edstica descriptiva utilitza mesures per descriure el fenomen que s&#8217;estudia. Les mesures m\u00e9s utilitzades s\u00f3n:<\/p>\n<p><strong>a) La moda (Mo)<\/strong><\/p>\n<p>\u00c9s el valor que es presenta amb major freq\u00fc\u00e8ncia en una distribuci\u00f3. Pot haver-n\u2019hi m\u00e9s de dues. Quan n\u2019hi ha una, parlem de distribuci\u00f3 \u00abunimodal\u00bb, i quan n\u2019hi ha dues ens referim a \u00abbimodals\u00bb.<\/p>\n<p>Un exemple \u00e9s:<\/p>\n<div class=\"tabletitle\"><p>Grau d&#8217;acord entre els pares per promoure la lectura dels diaris impresos entre els adolescents de dotze a disset anys:<\/p>\n<\/div>\n<table>\n<tbody>\n<tr>\n<td width=\"180\"><strong>Grau d&#8217;acord<\/strong><\/td>\n<td width=\"113\"><strong>Freq\u00fc\u00e8ncies<\/strong><\/td>\n<\/tr>\n<tr>\n<td width=\"180\">Molt d&#8217;acord<\/td>\n<td width=\"113\">20<\/td>\n<\/tr>\n<tr>\n<td width=\"180\"><strong>Bastant d&#8217;acord<\/strong><\/td>\n<td width=\"113\"><strong>60<\/strong><\/td>\n<\/tr>\n<tr>\n<td width=\"180\">Indiferent<\/td>\n<td width=\"113\">10<\/td>\n<\/tr>\n<tr>\n<td width=\"180\">Poc d&#8217;acord<\/td>\n<td width=\"113\">8<\/td>\n<\/tr>\n<tr>\n<td width=\"180\">Molt en desacord<\/td>\n<td width=\"113\">2<\/td>\n<\/tr>\n<tr>\n<td width=\"180\"><strong>Total<\/strong> (<em>N<\/em>)<\/td>\n<td width=\"113\">100<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>En aquest cas, la moda \u00e9s la categoria \u00abbastant d&#8217;acord\u00bb, triada per seixanta persones.<\/p>\n<p>Si ens referim a <strong>dades agrupades<\/strong>, la moda ser\u00e0 el punt mitj\u00e0 de l&#8217;interval en qu\u00e8 estigui la moda. El punt mitj\u00e0 s&#8217;obt\u00e9 sumant els dos extrems i dividint-los entre dos.<\/p>\n<p>Un exemple \u00e9s:<\/p>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple<\/strong><\/p>\n<div class=\"tabletitle\"><p>Nombre d&#8217;hores setmanals que els estudiants de primer de Comunicaci\u00f3 dediquen a xatejar amb WhatsApp:<\/p>\n<\/div>\n<table>\n<tbody>\n<tr>\n<td><strong>Hores setmanals<\/strong><\/td>\n<td><strong>Freq\u00fc\u00e8ncies<\/strong><\/td>\n<\/tr>\n<tr>\n<td>0-1<\/td>\n<td>18<\/td>\n<\/tr>\n<tr>\n<td>2-3<\/td>\n<td>10<\/td>\n<\/tr>\n<tr>\n<td>4-5<\/td>\n<td>12<\/td>\n<\/tr>\n<tr>\n<td><strong>6-7<\/strong><\/td>\n<td><strong>40<\/strong><\/td>\n<\/tr>\n<tr>\n<td>Total<\/td>\n<td>80<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Mo: 6,5<\/p>\n<p>En aquest cas la moda \u00e9s 6,5. La xifra s\u2019obt\u00e9 de sumar 6 i 7 i dividir-ho entre 2.<\/p>\n<p>\n<\/div>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple de variable quantitativa discreta<\/strong><\/p>\n<p>Calcula la moda de les dades seg\u00fcents: 1,1,2,3,3,3,5,7,7,8,9.<\/p>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<p>El valor de la variable que es repeteix m\u00e9s \u00e9s 3; per tant, la moda \u00e9s 3.<\/p>\n<p>Si la variable \u00e9s quantitativa cont\u00ednua, la moda tampoc no ha de ser \u00fanica.<\/p>\n<p>Per a cada interval\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-754\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn013-3.png\" alt=\"\" width=\"12\" height=\"24\" \/>\u00a0m\u00e0xim s&#8217;aplica la f\u00f3rmula seg\u00fcent:<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-753\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn009-1.png\" alt=\"\" width=\"230\" height=\"52\" srcset=\"\/wp-content\/uploads\/2020\/11\/Eqn009-1.png 598w, \/wp-content\/uploads\/2020\/11\/Eqn009-1-300x68.png 300w\" sizes=\"(max-width: 230px) 100vw, 230px\" \/><\/p>\n<p>Essent:<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-755\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn010-1.png\" alt=\"\" width=\"70\" height=\"54\" \/><\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-756\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn011-3.png\" alt=\"\" width=\"20\" height=\"32\" \/>\u00a0\u00e9s l&#8217;extrem inferior de l&#8217;interval modal,\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-757\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn012-2.png\" alt=\"\" width=\"12\" height=\"24\" \/>\u00a0l&#8217;extrem superior de l&#8217;interval anterior, <em>n<\/em> la grand\u00e0ria de la mostra,\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-758\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn013-4.png\" alt=\"\" width=\"12\" height=\"24\" \/>\u00a0la freq\u00fc\u00e8ncia absoluta corregida corresponent a l&#8217;interval modal,\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-759\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn014-2.png\" alt=\"\" width=\"20\" height=\"24\" \/>\u00a0la freq\u00fc\u00e8ncia absoluta corregida corresponent a l&#8217;interval anterior al modal,\u00a0i\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-760\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn015-2.png\" alt=\"\" width=\"23\" height=\"24\" \/>\u00a0la freq\u00fc\u00e8ncia absoluta corregida corresponent a l&#8217;interval posterior a l&#8217;interval modal.<\/p>\n<p>En el cas que tots els intervals tinguin la mateixa amplitud es verifica que\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-761\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn016-2.png\" alt=\"\" width=\"65\" height=\"24\" \/>.<\/p>\n<p>\n<\/div>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple de variable quantitativa cont\u00ednua amb intervals de la mateixa amplitud<\/strong><\/p>\n<p>Calcula la moda de les dades seg\u00fcents:<\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-772\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn016-01.png\" alt=\"\" width=\"8\" height=\"21\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[1,2)<\/td>\n<td width=\"189\">15<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[2,3)<\/td>\n<td width=\"189\">25<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[3,4)<\/td>\n<td width=\"189\">10<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<p>Com que els intervals tenen la mateixa amplitud, no fa falta calcular\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-758\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn013-4.png\" alt=\"\" width=\"12\" height=\"24\" \/>. L&#8217;interval modal \u00e9s el [2,3), perqu\u00e8 \u00e9s el de m\u00e9s freq\u00fc\u00e8ncia absoluta, i apliquem la f\u00f3rmula per determinar el valor de la moda:<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-762\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn019-1.png\" alt=\"\" width=\"250\" height=\"60\" srcset=\"\/wp-content\/uploads\/2020\/11\/Eqn019-1.png 601w, \/wp-content\/uploads\/2020\/11\/Eqn019-1-300x72.png 300w\" sizes=\"(max-width: 250px) 100vw, 250px\" \/><\/p>\n<p>\n<\/div>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple de variable quantitativa cont\u00ednua amb intervals d&#8217;amplitud diferent<\/strong><\/p>\n<p>Calcula la moda de les dades seg\u00fcents:<\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-772\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn016-01.png\" alt=\"\" width=\"8\" height=\"21\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[1,2)<\/td>\n<td width=\"189\">15<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[2,4)<\/td>\n<td width=\"189\">25<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[4,7)<\/td>\n<td width=\"189\">10<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<p>Com que els intervals no tenen la mateixa amplitud, fa falta calcular <img loading=\"lazy\" class=\"alignnone wp-image-758\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn013-4.png\" alt=\"\" width=\"12\" height=\"24\" \/>.<\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-772\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn016-01.png\" alt=\"\" width=\"8\" height=\"21\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-758\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn013-4.png\" alt=\"\" width=\"12\" height=\"24\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[1,2)<\/td>\n<td width=\"189\">15<\/td>\n<td width=\"189\">15<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[2,4)<\/td>\n<td width=\"189\">25<\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-767\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn019-01.png\" alt=\"\" width=\"50\" height=\"31\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[4,7)<\/td>\n<td width=\"189\">10<\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-768\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn020.png\" alt=\"\" width=\"45\" height=\"31\" \/><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Com que el <img loading=\"lazy\" class=\"alignnone wp-image-758\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn013-4.png\" alt=\"\" width=\"12\" height=\"24\" \/>\u00a0m\u00e0xim \u00e9s 15, l&#8217;interval modal \u00e9s el [1,2), i apliquem la f\u00f3rmula per determinar el valor de la moda:<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-769\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn021.png\" alt=\"\" width=\"230\" height=\"53\" srcset=\"\/wp-content\/uploads\/2020\/11\/Eqn021.png 625w, \/wp-content\/uploads\/2020\/11\/Eqn021-300x69.png 300w\" sizes=\"(max-width: 230px) 100vw, 230px\" \/><\/p>\n<p>\n<\/div>\n<p><strong>b) La mediana<\/strong><\/p>\n<p>\u00c9s el punt o valor num\u00e8ric que deixa la meitat de les puntuacions per sota. S&#8217;ordenen de m\u00e9s petit a m\u00e9s gran tots els casos i es troba el cas que estigui en el punt mitj\u00e0. Quan s\u00f3n distribucions imparelles, la mediana est\u00e0 en tot el centre. Es calcula de la manera seg\u00fcent:<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-1169\" src=\"\/wp-content\/uploads\/2020\/12\/Eqn022_CA.png\" alt=\"\" width=\"130\" height=\"42\" srcset=\"\/wp-content\/uploads\/2020\/12\/Eqn022_CA.png 318w, \/wp-content\/uploads\/2020\/12\/Eqn022_CA-300x97.png 300w\" sizes=\"(max-width: 130px) 100vw, 130px\" \/><\/p>\n<div class=\"featured featured-grey\"><\/p>\n<p>Hores de consum de Facebook de cinc estudiants:<\/p>\n<p>5\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 6\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <strong>7\u00a0<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 8\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 9<\/p>\n<p>La mediana \u00e9s 7, exactament al centre. Obtenim el mateix nombre si apliquem la f\u00f3rmula.<\/p>\n<p>Si la distribuci\u00f3 \u00e9s parella, per exemple, el nombre d&#8217;hores setmanals que sis amics veuen Netflix:<\/p>\n<p>18\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 20\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 28\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 30\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 42\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 44<\/p>\n<p>Les dades num\u00e8riques que deixen per sobre i per sota d\u2019un nombre igual de dades s\u00f3n 28 i 30. Se sumen i es divideixen per 2:<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-771\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn023.png\" alt=\"\" width=\"80\" height=\"51\" \/><\/p>\n<p>\n<\/div>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple de variable quantitativa discreta<\/strong><\/p>\n<p>Calcula la mediana de les dades seg\u00fcents: 1,1,2,3,3,3,5,7,7,8,9.<\/p>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-774\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn024.png\" alt=\"\" width=\"12\" height=\"17\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-775\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn025.png\" alt=\"\" width=\"14\" height=\"19\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">1<\/td>\n<td width=\"189\">2<\/td>\n<td width=\"189\">2<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">2<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">3<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">3<\/td>\n<td width=\"189\">3<\/td>\n<td width=\"189\">6<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">5<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">7<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">7<\/td>\n<td width=\"189\">2<\/td>\n<td width=\"189\">9<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">8<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">10<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">9<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">11<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-776\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn026.png\" alt=\"\" width=\"60\" height=\"54\" \/><\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-777\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn027.png\" alt=\"\" width=\"50\" height=\"37\" \/>\u00a0. Tal com veiem en la columna de les freq\u00fc\u00e8ncies absolutes acumulades, en el valor 6 \u00e9s on ens passem per primera vegada de 5,5; per tant, el valor de la mediana \u00e9s 3.<\/p>\n<p>D&#8217;una altra manera, ens quedem amb el que ocupa el lloc central despr\u00e9s d\u2019ordenar les dades.<\/p>\n<p><strong>1,1,2,3,3,3,5,7,7,8,9<\/strong><\/p>\n<p>\n<\/div>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple<\/strong><\/p>\n<p>Calcula la mediana de les dades seg\u00fcents: 1,1, 2,3, 3,3, 5,7, 7,8, 9,9.<\/p>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-774\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn024.png\" alt=\"\" width=\"12\" height=\"17\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-775\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn025.png\" alt=\"\" width=\"14\" height=\"19\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">1<\/td>\n<td width=\"189\">2<\/td>\n<td width=\"189\">2<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">2<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">3<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">3<\/td>\n<td width=\"189\">3<\/td>\n<td width=\"189\">6<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">5<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">7<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">7<\/td>\n<td width=\"189\">2<\/td>\n<td width=\"189\">9<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">8<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">10<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">9<\/td>\n<td width=\"189\">2<\/td>\n<td width=\"189\">12<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Els dos valors centrals, despr\u00e9s d\u2019ordenar les dades de la variable de m\u00e9s petita a m\u00e9s gran, s\u00f3n el 3 i el 5:<\/p>\n<p>1,1, 2,3, 3,3, 5,7, 7,8, 9,9<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-778\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn028.png\" alt=\"\" width=\"80\" height=\"51\" \/><\/p>\n<p>D&#8217;una altra manera, ens fixem en quin valor de la variable t\u00e9 com a freq\u00fc\u00e8ncia absoluta la meitat de les dades i calculem la mitjana d&#8217;aquest valor i del seg\u00fcent.<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-779\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn029.png\" alt=\"\" width=\"62\" height=\"56\" \/><\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-780\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn030.png\" alt=\"\" width=\"40\" height=\"52\" \/><\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-781\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn031.png\" alt=\"\" width=\"95\" height=\"55\" \/><\/p>\n<p>Si la variable \u00e9s quantitativa cont\u00ednua, la mediana estar\u00e0 en aquell interval en la freq\u00fc\u00e8ncia absoluta acumulada del qual passem de menys de la meitat a m\u00e9s de la meitat de les dades, ordenades pr\u00e8viament de m\u00e9s petita a m\u00e9s gran. Si la freq\u00fc\u00e8ncia absoluta acumulada coincideix amb la meitat de les dades, la mediana \u00e9s l&#8217;extrem de l&#8217;interval, i en cas contrari s&#8217;aplica la f\u00f3rmula seg\u00fcent:<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-783\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn032.png\" alt=\"\" width=\"186\" height=\"60\" srcset=\"\/wp-content\/uploads\/2020\/11\/Eqn032.png 435w, \/wp-content\/uploads\/2020\/11\/Eqn032-300x97.png 300w\" sizes=\"(max-width: 186px) 100vw, 186px\" \/>\u00a0essent\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-785\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn036-1.png\" alt=\"\" width=\"25\" height=\"23\" \/>\u00a0l&#8217;extrem inferior de l&#8217;interval mitj\u00e0,\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-757\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn012-2.png\" alt=\"\" width=\"13\" height=\"26\" \/>\u00a0l&#8217;extrem superior de l&#8217;interval anterior, <em>n<\/em> la grand\u00e0ria de la mostra,\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-775\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn025.png\" alt=\"\" width=\"15\" height=\"20\" \/>\u00a0la freq\u00fc\u00e8ncia absoluta acumulada que es passa del valor de la meitat de les dades, i finalment\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-786\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn036-01.png\" alt=\"\" width=\"24\" height=\"21\" \/>\u00a0la freq\u00fc\u00e8ncia absoluta acumulada anterior a la seleccionada.<\/p>\n<p>\n<\/div>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple de variable quantitativa cont\u00ednua<\/strong><\/p>\n<p>Calcula la mediana de les dades seg\u00fcents:<\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-772\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn016-01.png\" alt=\"\" width=\"8\" height=\"21\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[1,2)<\/td>\n<td width=\"189\">15<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[2,3)<\/td>\n<td width=\"189\">25<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[3,4)<\/td>\n<td width=\"189\">10<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-772\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn016-01.png\" alt=\"\" width=\"8\" height=\"21\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-775\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn025.png\" alt=\"\" width=\"14\" height=\"19\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[1,2)<\/td>\n<td width=\"189\">15<\/td>\n<td width=\"189\">15<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[2,3)<\/td>\n<td width=\"189\">25<\/td>\n<td width=\"189\">40<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[3,4)<\/td>\n<td width=\"189\">10<\/td>\n<td width=\"189\">50<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Com que el nombre total de dades \u00e9s 50, i la meitat d&#8217;aquestes s\u00f3n 25, l&#8217;interval mitj\u00e0 seria [2,3), perqu\u00e8 la seva freq\u00fc\u00e8ncia absoluta acumulada \u00e9s de 40 (la primera freq\u00fc\u00e8ncia absoluta acumulada que passa de 25).<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-787\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn037.png\" alt=\"\" width=\"200\" height=\"65\" srcset=\"\/wp-content\/uploads\/2020\/11\/Eqn037.png 427w, \/wp-content\/uploads\/2020\/11\/Eqn037-300x98.png 300w\" sizes=\"(max-width: 200px) 100vw, 200px\" \/><\/p>\n<p>\n<\/div>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple<\/strong><\/p>\n<p>Calcula la mediana de les dades seg\u00fcents:<\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-772\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn016-01.png\" alt=\"\" width=\"8\" height=\"21\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[1,2)<\/td>\n<td width=\"189\">25<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[2,3)<\/td>\n<td width=\"189\">5<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[3,4)<\/td>\n<td width=\"189\">20<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-772\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn016-01.png\" alt=\"\" width=\"8\" height=\"21\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-775\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn025.png\" alt=\"\" width=\"14\" height=\"19\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[1,2)<\/td>\n<td width=\"189\">25<\/td>\n<td width=\"189\">25<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[2,3)<\/td>\n<td width=\"189\">5<\/td>\n<td width=\"189\">30<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[3,4)<\/td>\n<td width=\"189\">20<\/td>\n<td width=\"189\">50<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Com que el nombre total de dades \u00e9s 50, i la meitat d&#8217;aquestes \u00e9s 25, l&#8217;interval mitj\u00e0 seria [1,2), perqu\u00e8 la seva freq\u00fc\u00e8ncia absoluta acumulada \u00e9s de 25 i el valor de la mediana coincideix en aquest cas amb l&#8217;extrem superior de l&#8217;interval, \u00e9s a dir,\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-788\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn038.png\" alt=\"\" width=\"40\" height=\"20\" \/>.<\/p>\n<p>\n<\/div>\n<p><strong>c) La mitjana aritm\u00e8tica i la mitjana ponderada<\/strong><\/p>\n<p>La mitjana aritm\u00e8tica ens informa de la mesura de tend\u00e8ncia central d&#8217;un grup de dades. S&#8217;obt\u00e9 quan se sumen tots els valors i es divideix pel total de casos.<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-790\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn039.png\" alt=\"\" width=\"60\" height=\"59\" \/><\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-792\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn040.png\" alt=\"\" width=\"15\" height=\"23\" \/> \u00c9s la mitjana aritm\u00e8tica.<br \/>\n\u03a3 \u00c9s el s\u00edmbol de la lletra grega sigma en maj\u00fascules i fa refer\u00e8ncia a la suma.<br \/>\nx \u00c9s cadascun dels valors observats.<\/p>\n<p><em>N<\/em> \u00e9s el nombre total de puntuacions d&#8217;una distribuci\u00f3.<\/p>\n<p>Per exemple, si volem obtenir la mitjana de les notes (6, 5 i 7) de tres treballs de l\u2019assignatura Metodologies de la recerca d&#8217;un alumne:<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-791\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn040-01.png\" alt=\"\" width=\"85\" height=\"50\" \/><\/p>\n<p>Si l&#8217;\u00faltima d&#8217;aquestes notes tingu\u00e9s tres vegades m\u00e9s valor que les altres, parlar\u00edem de la mitjana ponderada i es calcularia aix\u00ed:<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-793\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn041.png\" alt=\"\" width=\"130\" height=\"54\" srcset=\"\/wp-content\/uploads\/2020\/11\/Eqn041.png 340w, \/wp-content\/uploads\/2020\/11\/Eqn041-300x124.png 300w\" sizes=\"(max-width: 130px) 100vw, 130px\" \/><\/p>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple de mitjana aritm\u00e8tica<\/strong><\/p>\n<p>Es van seleccionar cinc persones i se&#8217;ls va preguntar el nombre d&#8217;hores al dia que passaven connectats a qualsevol xarxa social. Les dades van ser les seg\u00fcents: 2,1; 5,3; 1,4; 4,6; 0,7. Calculeu la mitjana del temps en hores al dia que van passar connectats a les xarxes socials.<\/p>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<p><em>X<\/em> \u2261 temps en hores de connexi\u00f3 al dia a xarxes socials.<\/p>\n<p>Es tracta d\u2019una variable quantitativa cont\u00ednua.<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-1170\" src=\"\/wp-content\/uploads\/2020\/12\/Eqn042_CA.png\" alt=\"\" width=\"320\" height=\"38\" srcset=\"\/wp-content\/uploads\/2020\/12\/Eqn042_CA.png 715w, \/wp-content\/uploads\/2020\/12\/Eqn042_CA-300x36.png 300w\" sizes=\"(max-width: 320px) 100vw, 320px\" \/><\/p>\n<p>\n<\/div>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple de mitjana ponderada<\/strong><\/p>\n<p>Amb les dades de l&#8217;apartat anterior, i tenint en compte que hem ponderat cada subjecte respectivament amb els valors 1, 2, 3, 4, i 5, s&#8217;han donat aquests valors en concordan\u00e7a amb les edats dels enquestats: com menys edat m\u00e9s pes. Calculeu la mitjana geom\u00e8trica del temps en hores al dia que van passar connectats a xarxes socials.<\/p>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-1171\" src=\"\/wp-content\/uploads\/2020\/12\/Eqn043_CA.png\" alt=\"\" width=\"450\" height=\"42\" srcset=\"\/wp-content\/uploads\/2020\/12\/Eqn043_CA.png 905w, \/wp-content\/uploads\/2020\/12\/Eqn043_CA-300x28.png 300w, \/wp-content\/uploads\/2020\/12\/Eqn043_CA-768x72.png 768w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><\/p>\n<p>\n<\/div>\n<p><strong>d) Percentils<\/strong><\/p>\n<p>\u00c9s una mesura de posici\u00f3 no central que ens diu la posici\u00f3 d&#8217;un valor respecte als altres. Per exemple, si un valor est\u00e0 en el quartil 2 (Q2), ens diu que dues quartes parts de les dades s\u00f3n iguals o menors que aquest valor.<\/p>\n<p>Si un valor t\u00e9 un percentil 20 (P20), el 20 % de les dades restants tindr\u00e0 el seu mateix valor o menys.<\/p>\n<p>Per exemple, estem estudiant les hores de consum diari d&#8217;Instagram de joves universitaris entre 18 i 22 anys. Si el consum de quatre hores est\u00e0 en el percentil 80 (P80), el 80 % d&#8217;aquests estudiants utilitza Instagram durant quatre hores o menys.<\/p>\n<p>El percentil <em>k<\/em> \u00e9s el primer valor de la variable que deixa inferiors o iguals a ell les <img loading=\"lazy\" class=\"alignnone wp-image-796\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn044.png\" alt=\"\" width=\"25\" height=\"34\" \/>\u00a0parts de les observacions. Es calcula el valor\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-796\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn044.png\" alt=\"\" width=\"25\" height=\"34\" \/>\u00a0i es tria l\u2019interval la freq\u00fc\u00e8ncia absoluta acumulada del qual \u00e9s igual o major que aquest valor.<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-797\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn045.png\" alt=\"\" width=\"195\" height=\"71\" srcset=\"\/wp-content\/uploads\/2020\/11\/Eqn045.png 478w, \/wp-content\/uploads\/2020\/11\/Eqn045-300x109.png 300w\" sizes=\"(max-width: 195px) 100vw, 195px\" \/><\/p>\n<p>Hi ha cent percentils. Per definici\u00f3, <img loading=\"lazy\" class=\"alignnone wp-image-798\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn046.png\" alt=\"\" width=\"55\" height=\"23\" \/><\/p>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple de variable quantitativa discreta<\/strong><\/p>\n<p>Calcula el percentil 1 de les dades seg\u00fcents: 1,1, 2,3, 3,3, 5,7,7,8,9.<\/p>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-774\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn024.png\" alt=\"\" width=\"12\" height=\"17\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-775\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn025.png\" alt=\"\" width=\"14\" height=\"19\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">1<\/td>\n<td width=\"189\">2<\/td>\n<td width=\"189\">2<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">2<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">3<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">3<\/td>\n<td width=\"189\">3<\/td>\n<td width=\"189\">6<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">5<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">7<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">7<\/td>\n<td width=\"189\">2<\/td>\n<td width=\"189\">9<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">8<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">10<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">9<\/td>\n<td width=\"189\">1<\/td>\n<td width=\"189\">11<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-776\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn026.png\" alt=\"\" width=\"60\" height=\"54\" \/><\/p>\n<p>El percentil 1 deixar\u00e0 per sota seu la cent\u00e8sima part de les observacions.<\/p>\n<p>Calculem el percentil 1.<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-800\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn048.png\" alt=\"\" width=\"60\" height=\"29\" \/>. Si ens fixem en la columna de les freq\u00fc\u00e8ncies absolutes acumulades, la primera que passa d&#8217;aquest valor \u00e9s 2, que es correspon amb el valor de la variable 1; per tant:\u00a0<img loading=\"lazy\" class=\"alignnone wp-image-801\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn049.png\" alt=\"\" width=\"30\" height=\"30\" \/><\/p>\n<p>\n<\/div>\n<div class=\"featured featured-grey\"><\/p>\n<p><strong>Exemple de variable quantitativa cont\u00ednua<\/strong><\/p>\n<p>Calcula el percentil 97 tenint en compte les dades seg\u00fcents:<\/p>\n<table>\n<tbody>\n<tr>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-772\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn016-01.png\" alt=\"\" width=\"8\" height=\"21\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-773\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn017.png\" alt=\"\" width=\"12\" height=\"16\" \/><\/td>\n<td width=\"189\"><img loading=\"lazy\" class=\"alignnone wp-image-775\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn025.png\" alt=\"\" width=\"14\" height=\"19\" \/><\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[1,2)<\/td>\n<td width=\"189\">15<\/td>\n<td width=\"189\">15<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[2,3)<\/td>\n<td width=\"189\">25<\/td>\n<td width=\"189\">40<\/td>\n<\/tr>\n<tr>\n<td width=\"189\">[3,4)<\/td>\n<td width=\"189\">10<\/td>\n<td width=\"189\">50<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><strong>Soluci\u00f3<\/strong><\/p>\n<p>El percentil 97 deixar\u00e0 el 97 % de les observacions per sota seu.<\/p>\n<p>En aquest cas particular, com que hi ha cinquanta dades, deixar\u00e0 per sota:<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-802\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn050.png\" alt=\"\" width=\"95\" height=\"44\" \/><\/p>\n<p>Si ens fixem en la columna de les freq\u00fc\u00e8ncies absolutes acumulades, la primera que passa d&#8217;aquest valor \u00e9s 50; per tant, ens quedem amb l&#8217;interval [3,4) i apliquem la f\u00f3rmula.<\/p>\n<p><img loading=\"lazy\" class=\"alignnone wp-image-803\" src=\"\/wp-content\/uploads\/2020\/11\/Eqn051.png\" alt=\"\" width=\"220\" height=\"72\" srcset=\"\/wp-content\/uploads\/2020\/11\/Eqn051.png 566w, \/wp-content\/uploads\/2020\/11\/Eqn051-300x98.png 300w\" sizes=\"(max-width: 220px) 100vw, 220px\" \/><\/p>\n<p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>L&#8217;estad\u00edstica descriptiva utilitza mesures per descriure el fenomen que s&#8217;estudia. Les mesures m\u00e9s utilitzades s\u00f3n: a) La moda (Mo) \u00c9s el valor que es presenta amb major freq\u00fc\u00e8ncia en una distribuci\u00f3. Pot haver-n\u2019hi m\u00e9s de dues. Quan n\u2019hi ha una, parlem de distribuci\u00f3 \u00abunimodal\u00bb, i quan n\u2019hi ha dues ens referim a \u00abbimodals\u00bb. Un exemple [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":[],"_links":{"self":[{"href":"http:\/\/tecniques-metodes-recerca.recursos.uoc.edu\/ca\/wp-json\/wp\/v2\/pages\/1167"}],"collection":[{"href":"http:\/\/tecniques-metodes-recerca.recursos.uoc.edu\/ca\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"http:\/\/tecniques-metodes-recerca.recursos.uoc.edu\/ca\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"http:\/\/tecniques-metodes-recerca.recursos.uoc.edu\/ca\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/tecniques-metodes-recerca.recursos.uoc.edu\/ca\/wp-json\/wp\/v2\/comments?post=1167"}],"version-history":[{"count":2,"href":"http:\/\/tecniques-metodes-recerca.recursos.uoc.edu\/ca\/wp-json\/wp\/v2\/pages\/1167\/revisions"}],"predecessor-version":[{"id":1172,"href":"http:\/\/tecniques-metodes-recerca.recursos.uoc.edu\/ca\/wp-json\/wp\/v2\/pages\/1167\/revisions\/1172"}],"wp:attachment":[{"href":"http:\/\/tecniques-metodes-recerca.recursos.uoc.edu\/ca\/wp-json\/wp\/v2\/media?parent=1167"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}