Quadrats are square, gridded frames used to provide a random quantitative sample to estimate a population size
Transects are used to show the change in distribution on a line between two points using regular quadrat measurements along it
Validity
Investigations about the distribution of organisms can be difficult since they are often carried out over a long time and not all variables can be controlled
A valid investigation is one in which all possible variables are controlled
A repeatable measurement is one by which the same method & equipment results in the same measurement
A reproducible measurement is one by which someone else repeating the investigation is able to obtain the same results
Sample size is related to validity, reproducibility and repeatability, since if sample size is too small, it may not be representative, so the larger the sample size, the more trustworthy it is