Stats Question!?

Question

Statistics Question?? One tailed or Two tailed test?
Scores on standardized intelligence tests are normally distributed with m = 100 and s = 15. You think that experience is especially important in determining intelligence and that an enriched environment can increase a child’s IQ test scores. You randomly select a sample of 30 children to participate in an enrichment program. After their experience in the enrichment program, the IQ scores for the sample is M = 110.

a. Is there evidence that your enrichment program increased IQ test scores?

b. In answering part a., did you use a one-tailed test or a two-tailed test? Justify your choice of a one-tailed or two-tailed test. How did this choice influence your conclusion?

c. Comment on the effect size obtained with your enrichment program.

- I used a Two tailed test to calculate (a) and Cohen's d for part (c) Now I am wondering if I should have used a One tail test???

Answer 1

I would have used a one-tailed test as the question asks if there is evidence for an increase. If it had asked if there was a difference (or an effect) I would have used a two-tailed test.

I am not sure what the result you would have got using a two-tailed test (cant be bothered calculating it :)), but you may find that a two-tailed test may not give you a significant result as your result would have to be even more extreme than in a one-tailed test (this answers the conclusion part if you failed to reject the null).

But you may be fine in using the two-tailed test aslong as you justify why you used it and realise that if your result was not significant it was because two-tailed tests require a more extreme value than a one-tailed test before you can reject the null hypothesis.