# Pre-Post Test Improvement

1. Mar 28, 2009

### craig100

Hello There,

I'm after a little advice; I have data collected during experimentation that was designed to determine the improvement of two groups of subjects between a pre and a post-test. The results are as follows:
Pre & Post test numbers represent the number of errors made
Group 1:
Code (Text):

Pre|Post|Improvement
0   0   0
1   0   1
5   4   1
12  0   12
2   0   2
7   6   1
0   0   0
13  0   13

Group 2:
Code (Text):

Pre|Post|Improvement
0   1   -1
2   0   2
1   0   1
0   0   0
0   11  -11
5   23  -18

I'm applying the t-test to determine if there is a statistical difference between the means(of the improvement columns).

My question is really simple(hopefully)... in group 2, should I be recording improvement as a negative number? or simply zero?

Using zero will not give me a significant statistical difference between the means of group 1 and group 2... using the negative numbers does.

Also, if I'm keeping the negative improvement numbers, is it okay to represent improvement as a negative percentage?

I've not done much with statistics, so would appreciate your input.

Thanks,
Craig.

2. Mar 29, 2009