Sampling methods should ensure the randomness of the sample from the targeted population, and several methods are used in order to achieve this. Random sampling is the simplest form, and with this technique, all members of the population have an equal chance of being chosen. Systematic sampling allows researchers to choose from a population according to pre-defined rules, for example, choosing every tenth individual. Stratified sampling creates groups from a population with factors they have in common, and then selects a sample from the members of these groups. The three forms mentioned above are probability samplings, but there are also non-probability samplings as well, e.g. when we choose the most convenient or easiest sample, and judgement samples, where the researcher chooses members of a sample based on his or her knowledge about the population.