How Many Samples are Enough When Data are Unbalanced?

Yıl 2005, Cilt: 11 Sayı: 03, 225 - 228, 01.08.2005

Mehmet Mendeş

Öz

A crucial component of the design of a study is the number of participants or observations sample size required. Taking too many samples will waste time and resources, both in collecting and analyzing the data. On the other hand, taking too small samples can make the whole study meaningless or lead to errors in interpritation. Equal group sizes are preferable. But, this is not always the case in practice. The aim of this study is to clarify some of the key issues regarding sample size and power 80 % when data are unbalanced. For this aim, a simulation study was conducted. At the end of the 50,000-simulation trial it was seen that there are many different sample size combinations that make it possible to reach around 80% test power. On the other hand, as the numbers of observations were getting more different, we needed more observations to reach around 80 % test power. For instance, the test power we reached for the 16 observations in each group n=16:16:16 , total 48 observations, we can only reach with 72 observations when sample sizes were unequal n=12, 30, 30 and n=12: 24: 36 . As the variances were getting more heterogenous, the effect of unbalanced data on test power was getting more obvious

Anahtar Kelimeler

Optimum sample size, test power, effect size, unbalanced data

Dengesiz Verilerle Çalışılması Durumunda Gruplardaki Gözlem Sayıları Kaç Olmalıdır?

Öz

Deneme planlamasında en önemli aşamalardan birisi, gerekli olan örnek hacminin belirlenmesidir. Örnek hacminin gereğinden fazla olması kaynakların israfına neden olmaktadır. Gereğinden az olması durumunda ise parametre tahminlerinde oldukça büyük sapmalar meydana gelmekte ve karşılaştırılacak muamele grup ortalamaları arasında gerçekte var olan farklılıklar ortaya konulamamaktadır. Karşılaştırılacak gruplardaki gözlem sayılarının eşit olması istenen bir durumdur. Ancak, uygulamada her zaman eşit hacimli örneklerle çalışmak mümkün olamamaktadır. Bu çalışmada, dengesiz denemelerin söz konusu olması durumunda hangi örnek hacmi kombinasyonlarının % 80’lik güç değerini sağlayabildiklerinin belirlenmesi amacıyla bir simülasyon çalışması yapılmıştır. Yapılan 50,000 simülasyon denemesi sonucunda, pek çok örnek hacmi kombinasyonu ile çalışılması durumunda % 80’lik güç değerine ulaşıldığı görülmüştür. Ancak, örnek hacimlerindeki dengesizliğin artması, araştırıcıyı daha fazla gözlem ile çalışmaya zorlamaktadır. Mesela varyanslar homojen iken n= 16, 16, 16 örnek hacmi kombinasyonu toplam 48 gözlem ile varılan güç değerine, dengesiz denemelerin söz konusu olması durumunda ancak n= 12, 30, 30 ve n= 12, 24, 36 toplam 72 gözlem örnek hacmi kombinasyonu ile çalışılması durumunda ulaşılmaktadır. Varyansların heterojenlik derecesinin artmasına paralel olarak örnek hacimlerindeki dengesizliklerin testin gücüne olan etkilerinin daha da belirginleştiği görülmüştür

Anahtar Kelimeler

Uygun örnek hacmi, testin gücü, etki büyüklüğü, dengesiz veriler

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