Statistics Definitions Part 2 Representative Sample For a sample to be representative of a population, it must possess the same characteristics as the population itself. It must provide the most accurate profile possible of the population. An appropriate representative sample is one in which the people who take part in the study are selected at random. When a sample is not representative, it is biased. Not representative = source of bias SAMPLING METHODS Random sampling: Each member of the population has equal chances of being chosen. When a population is homogeneous (same) in terms of the given variable and when it is possible to identify all of its members, the random sampling is ideal. SAMPLING METHODS Stratified sampling: In a case where the population is heterogeneous (different) and where membership in a group may affect the outcome, the sample is divided into subgroups called strata. Each stratum is represented according to the same ratio, both in the sample and in the population. Individuals in each stratum are selected at random. The sample is said to be stratified. SAMPLING METHODS Systematic sampling: This method consists of selecting a starting point at random and then always using the same method to select the other members of the sample. SAMPLING METHODS Cluster sample: The cluster method is used when the population is divided into groups. The sample is composed of all the members that are part of the clusters selected at random. Examples (dont write) What type of sampling method? A) There are 100 boys and 50 girls in a soccer league. A sample of 10 boys and 5 girls is chosen at random for a survey. B) The first person on each page of a phone book are phoned for an interview. SOURCES OF BIAS Inappropriate sampling methods. Errors of measurements A poorly posed questions that is not pertinent to the variable being studied. The involvement and self-interest of the people questioned A low response rate The conditions under which the data were collected and the attitude of the pollster COMPARING SAMPLES The accuracy of a poll increases if the size of the sample is increased, but not proportionally. Beyond a certain size, the increase in accuracy is no longer significant. The largest sample is not necessarily the most representative.