The peppered moth (Biston betularia) occurs in two types: peppered (speckled black and white) and melanic (black). A researcher wished to measure the proportion of melanic individuals in the peppered moth population in England, to examine how this proportion changed from year to year in the past. To accomplish this, she photographed all the peppered moth specimens available in museums and large private collections and grouped them by the year in which they had been collected. Based on this sample, she calculated the proportion of melanic individuals in every year.

The people who collected the specimens, she knew, would prefer to collect whichever type was rarest in any given year, since those would be the most valuable.

a. Can the specimens from any given year be considered a random sample from the moth population?

b. If not a random sample, what type of sample is it?

c. What type of error might be introduced by the sampling method when estimating the proportion of melanic moths?