Question

1. Suppose we are interested in determining if the density of sugar kelp is similar in subtidal regions north and south of Cape Cod. We go to 12 randomly selected sites in each region, and within each site we sample 25 randomly selected 4 by 4 meter plots. In each of the plots we count the number of sugar kelp stipes, and use a t-test to compare the two regions (north vs. south of the Cape). a. What’s the dependent variable, and what’s the predictor or independent variable? b. What’s the null hypothesis? c. What counts as a replicate for this comparison – what are number of replicates per region? d. OK, NOW what if we want to focus on just the sites south of the Cape, and compare each site to each other... how many replicates do we have for this comparison?

Answer 1

Dependent variable = Density of kelps

Independent variable = Collection site (North or south of Cape cod)

Null hypothesis: There is no difference in density between the two regions.

Replicates: In the first set, we collect 12 samples each from north and south sites.

The replicate refers to another 12 samples each from north and south sites.

Replicates for south of the cape:

We should collect 12 samples from south of the cape in the first set.

We should collect another 12 samples from south of the cape in the second set.

We should compare both these groups.

So, total 24 samples.