General prerequisites:

  1. Goal of the test is to "Compare two paired groups".
  2. Type of data is "Measurement (from Gaussian Population)"
Specific prerequisites:
  1. n > 30
  2. random sampling.
  3. The variables are continuous.
  4. Same group two measurements (pretest, post test).

How to think about results from an unpaired t test

The Paired Samples T Test compares the means of two variables. It computes the difference between the two variables for each case, and tests to see if the average difference is significantly different from zero.

Following is sample output of a paired samples T test. We compared the mean test scores before (pre-test) and after (post-test) the subjects completed a test preparation course. We want to see if our test preparation course improved people's score on the test.

First, we see the descriptive statistics for both variables.

The post-test mean scores are higher.

Next, we see the correlation between the two variables.

There is a strong positive correlation. People who did well on the pre-test also did well on the post-test.

Finally, we see the results of the Paired Samples T Test. Remember, this test is based on the difference between the two variables. Under "Paired Differences" we see the descriptive statistics for the difference between the two variables.

To the right of the Paired Differences, we see the T, degrees of freedom, and significance.

The T value = -2.171

We have 11 degrees of freedom

Our significance is .053

If the significance value is less than .05, there is a significant difference.
If the significance value is greater than .05 there is no significant difference.

Here, we see that the significance value is approaching significance, but it is not a significant difference. There is no difference between pre- and post-test scores. Our test preparation course did not help!

Türkçe

Bağımlı t testi ne zaman kullanılır?

Genel koşullar:

1. Amacımız "Eşlenmiş iki grubu yada bir gruba ait ön test ve son test sonuçlarını karşılaştırmak" olduğunda.

2. Veri türümüz "Normal dağılımdan alınan bir örneklem" ise.

Özel koşullar:

1. n > 30

2. Rasgele örneklem seçimi

3. Değişkenler sürekli.

4. Aynı grup iki ölçüm (Ön test, son test)

Bağımlı t testi nasıl yorumlanır?

t testi yapıldığında significance p değeri .05 ten küçük çıkarsa % 95 güven aralığında ölçümler arasında anlamlı bir fark vardır denir.

Örneğin yukarıdaki örnek ön test ve son test karşılaştırmasında p = .053 bulmuştur. Buna göre significance .05 ten küçük olmadığı için ön test ve son test arasında anlamlı bir fark bulunamamıştır. Yani verilen öğretim yararlı olmamıştır.

Yukarıdaki örnekte

T değeri = -2.171

Serbestlik derecesi 11

significance .053 tür.

These calculations are
Courtesy of Susan Archambault
Psychology Department, Wellesley College