Mean The sum of the values in a set of data divided by the total number of values in a set. Median The middle value or the mean of the middle values in a set of data when the data are arranged in numerical order. Mode The value or values that appear most often in a set of data. Standard Deviation A measure that is used to quantify the amount of variation or dispersion of a set of data values. Percentile The percentage of scores that fall at or below a given score. Population The entire group of objects or individuals considered for a survey. Sample A part of a population. Bias An error that results in a misrepresentation of members of a population. Observational Study Individuals are observed and no attempt is made to influence the results. Experiment An experiment or trial that uses controls, usually separating the subjects into one or more control groups and experimental groups Survey Used to collect information about a population. Simulation The use of mathematical model to recreate a situation with a known theoretical probability. Convenience Sample A sample of the most available subjects in the population used to obtain quick results. Voluntary Response Sample Members of the population have volunteered to participate. Simple Random Sample A sample in which every member of the population has an equal chance of being selected. Stratified Random Sample The population is divided into groups based on a common characteristic and then a random sample is chosen from each group Systematic Random Sample Members are put into order and the first member is chosen randomly and then every nth member is chosen after that Cluster Sample The population is divided into groups not based on any common characteristic and then a simple random of each group is chosen. Normal Distribution A continuous, symmetric, bell-shaped distribution of a random variable. Empirical Rule The 68-95-99.7 rule provides a quick estimate of the spread of data in a normal distribution given the mean and standard deviation. Z-Score The (positive or negative) number of standard deviations that a given data value is from the mean in a distribution. Theoretical Probability The number of ways that the event can occur divided by the total number of outcomes. Experimental Probability The ratio of the number of times an event occurs to the total number of trials or times the activity is performed.