The researcher also collected height data from 200 female sheep during both years but didn't notice a significant
difference in height. What could this data suggest?

The data suggests that there is no change in height of sheep because of

Hardy Weinberg equillibrium.

Explanation:

Data given:

heights of 200 sheeps remains the same for two years.

reason=?

There is no change in the phenotype of the sheep population during two years as there is no effect of environmental factors, no evolution or genetic drift in the population. Such population happens due to Hardy Weinberg equillibrium.

This principle says that there will be no genetic variation in a population when no evolution causing factors are there. These factors are environmental, mutation, genetic drift, natural selection or gene flow.  When no new alleles are added in the population it remains same.

Thus in case of the 200 sheeps it is assumed that they were in Hardy Weinberg equillibrium.