Understanding Sampling Methods and Experimental Design

Sample Survey

Some members of a group are studied, but not all.

Census

Every member of the group must be studied.

Sampling Error

The difference between the true population parameter and the statistic obtained from your sample.

Experiment

When the researcher measures the relationship between some variables and actively creates some change in one of those variables to examine its effect on the other variables.

Treatment

The change in the independent variable that we believe causes a change in the dependent variable.

Observational Study

Cannot show causation between two variables, but can show association.

Mean

The average value of a set of numbers, represented by the symbol 𝜇 (pronounced 'mew').

Standard Deviation

A measure of the amount of variation or dispersion in a set of values, represented by the symbol 𝜎 (pronounced 'sigma').

Population Parameter

The actual measure of the population for some variable.

Sample Statistic

A statistic that is calculated from a sample, represented by the symbol 𝑥¯ (pronounced 'X-bar').

Representative Sample

A sample that matches the population's characteristics in terms of attitude, behavior, and/or demographics.

Sampling Frame

The list of subjects or units in a population from which the sample is chosen.

Incorrect Sampling Frame

A type of bias that occurs when the sampling frame does not accurately represent the characteristics of the population.

Association

A relationship between two variables where one variable does not cause a change in the other.

Causation

A relationship where one variable directly causes a change in another variable.

Bias

A systematic error that leads to an incorrect conclusion.

Data Collection

The process of gathering information for analysis.

Variables

Any characteristics, numbers, or quantities that can be measured or counted.

Independent Variable

The variable that is changed or controlled in a scientific experiment.

Dependent Variable

The variable that is measured and affected in an experiment.

Researcher

A person who conducts studies to gather data and analyze it.

Survey

A method of gathering information from individuals, often used in observational studies.

Sampling Frame

Errors with the sampling frame occur when some group of the population does not have an equal opportunity of being represented.

Sample of Convenience

A sample that is over-represented compared to the total population, typically consisting of friends and family who share similar values, interests, and opinions.

Judgment Sampling

A type of sampling based on a subjective choice that is highly susceptible to bias, often influenced by the prejudices of the individual making the choice.

Quota Sampling

Involves including exact proportions of individuals from different subgroups in the sample, but is still subject to individual biases.

Volunteer Sampling

A common source of sampling bias where subjects choose to participate, often leading to a sample that does not reflect the true feelings of the entire population.

Simple Random Sample (SRS)

A method used to avoid bias and select a truly representative sample through randomization.

Systematic Sampling

A method where a starting point is chosen, and subjects are selected using a jump number or specific interval.

Stratified Random Sample

A sampling method where the population is divided into groups or 'strata' that have a meaningful relationship with the variable being studied.

Proportional Sampling

A sampling method where the population is divided into strata, and a simple random sample of a size proportional to the size of the stratum is selected from each stratum.

Cluster Samples

Sampling method that uses non-homogeneous groups where each cluster contains various types.

Strata

Homogeneous groups in a population that share some characteristic and respond differently to surveys.