Sampling is taking the measurement of individuals selected from the population of organisms being studied.
There are several reasons however, why the sampling group might not be representative:
1.Sampling bias, as in, the scientists selecting the individuals to sample might have been biased towards a certain kind, knowingly or unwittingly. For example, if the scientists needed to pick flowers, would they not pick flowers from muddy ground because it is dirty?
2. Chance. Even if there has been no or little sampling bias involved, chance might have made your selection unrepresentative. For example, once again, if you are picking flowers, you might just happen to pick the 50 tallest flowers in the field, which will make your results unrepresentative of the flowers as a whole.
Random sampling reduces the likelihood of sampling bias as it reduces human involvement. Random sampling is carried out by using coordinates generated y a computer and taking measurements at these intersections of your study area, after having made it into a grid
Chance cannot completely be removed from your results, however, it's effect can be minimized by:
1. Using a large sample size - reduces the probability of the results being unrepresentative
2. Analysis of data - If you just accept that chance will play a part in your results, then you can find ways of formulating statistical tests to determine the extent to which chance has played a role in your investigation.
Causes of variation
Causes of variation are genetic differences, and environmental factors.
Scientists have decided that only hair colour, eye colour, blood type and inherited illnesses are the only things that are solely controlled by your genes, everything else is contributed to by the environment.
Genetic variation is a result of:
1. Fusion of gametes - In sexual reproduction genes from the mother and father are passed on to the child, but only the predominant genes are selcted to be shown as the child's characteristics.
2. Meiosis - Meiosis is a form of nuclear division in which gametes are made. Genetic material is mixed up (due to crossing over etc) and so many gametes with different genes are formed.
3. Mutation - Sudden changes to genes may or may not be passed on to future generations.