What size trucks/deck should I get?

Appropriate sample size to get a significant result - Statistics, a-priori testing?

• I have done some reading about how to make sure I have enough participants for me to gain significant results for a trial I hope to run soon, but I am not sure if I'm heading in the right direction. It is completely possible that I've totally misinterpreted everything and am wrong. I need to work this out before getting further into the proposal for the research, as if I can't get a significant result then I doubt I'll get ethical approval! I've used the tool G*POWER to determine whether I will be able to get significant results with the small sample size I have to work with: 6-8. I understand the output from G*POWER to mean that if I intend to test for a difference of one full letter grade (here that difference would be 10%), with both groups being fairly homogeneous (SD of 2.5), I will be able to do so with 3 participants or more per group. If I test for the same difference with double the SD, I will need 7 participants per group. Is this correct? (output below) The test: t-test difference between independent groups (2 grps) A priori: compute required sample size Estimates: grp 1 mean: 80 (%, an A grade) grp 1 SD: 2.5 (quarter of a grade variation) grp 2 mean: 70 (%, B grade) grp 2 SD: 2.5 (quarter of a grade variation) = effect size: 4 Input: Tail(s) = One Effect size d = 4 α err prob = 0.05 Power (1-β err prob) = 0.95 Allocation ratio N2/N1 = 1 Output: Noncentrality parameter δ = 4.898979 Critical t = 2.131847 Df = 4 Sample size group 1 = 3 Sample size group 2 = 3 Total sample size = 6 Actual power = 0.988811 And with SD of 5 (half grade) for both groups (effect size 2): Input: Tail(s) = One Effect size d = 2 α err prob = 0.05 Power (1-β err prob) = 0.95 Allocation ratio N2/N1 = 1 Output: Noncentrality parameter δ = 3.741657 Critical t = 1.782288 Df = 12 Sample size group 1 = 7 Sample size group 2 = 7 Total sample size = 14 Actual power = 0.969582 If someone with experience in this area could please let me know if I am correct? Thanks

• Answer:

  having taken AP Statistics I would say get a sample size of at least 30 and that is barely cutting it 100 or more is recommended though not always possible. So although it might work I don't recommend it at all. Sorry. "66% of the rats showed improvement, the other rat died" Just be careful I would at least double your sample size to have about 15 participants in each group in order to have a smaller Margin of error and improve accuracy. Also watch out because some stats programs have significant errors.