I think a paired t-test is what you are after:
http://udel.edu/~mcdonald/statpaired.html
Phil
Hi,
I'm collecting data from a single group of 15 participants showing their improvement in a 5 stage levelled test before and after an intervention. I will have a before and after level for each participant (and therefore an improvement for each) and the nationally expected improvement in level with and without the intervention. It's been a while since I had to do anything like this so I'm a bit confused! I want to show that the intervention significantly increases the rate of improvement.... but what test should I do? (The closest I've found is an Independent Measures T-Test comparing a single group with an exernal measure but I REALLY don't know what I'm talking about!)
PLEASE HELP ME...
Thanks x
I think a paired t-test is what you are after:
http://udel.edu/~mcdonald/statpaired.html
Phil
Thank you so much.
Is this too good to be true? I put in the improvement result compared to the national expectation of result and it says it's 'extremely statisticaly significant' (see cut and paste below)! Have I done it wrong or has my research actually worked?! Can I say somthing like... 'The Paired T-test shows that the rate of progress after the intervention is significantly better than the expected rate of progress with no intervention at a 95%confidnece level' ?
I wish I'd remembered all this stuff!
Thanks again
Paired t test results
P value and statistical significance:
The two-tailed P value equals 0.0009
By conventional criteria, this difference is considered to be extremely statistically significant.
Confidence interval:
The mean of improvement minus prediction equals 0.900
95% confidence interval of this difference: From 0.441 to 1.359
Intermediate values used in calculations:
t = 4.2094
df = 14
standard error of difference = 0.214
Learn more:
GraphPad's web site includes portions of the manual for GraphPad Prism that can help you learn statistics. First, review the meaning of P values and confidence intervals. Next check whether you chose an appropriate test. Then learn how to interpret results from an unpaired or paired t test. These links include GraphPad's popular analysis checklists.
Review your data:
Group improvement prediction
Mean 1.400 0.500
SD 0.828 0.000
SEM 0.214 0.000
N 15 15
I do not wish to burst your bubble but I do not think you are interpreting this test correctly. The paired sample t-test compares too groups of data (both from the same sample) to determine if there is a difference. So the significant difference you are finding is that the intervention is leading to a significant improvement. However, it does not tell you whether your improvement was significantly better than the "expected" improvement. The best way to determine whether your intervention leads to a significantly better improvement is to get before and after data from two groups (one with your intervention and one without) and perform a repeated-measures 2 x 2 ANOVA.
Assuming the test statistics at the bottom of your post are correct, then you have done the math correctly. The results are highly significant. In fact, since you were trying to show that the mean of the test group is higher (rather than just different) than the national average, you can cut the p-value in half... not that it matters when it is that small already.
Congrats, your research shows what you wanted it to show.
Last edited by squareandrare; 05-08-2010 at 01:00 AM.
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