The Main Idea 

Sampling methods that are not statistically random are biased. That is, they inherently exclude certain members of the population from being selected in the sample.

The Main Idea

Simple random sampling assigns a number to every member of the population, then uses a random number generator to select a sample.

Systematic sampling assigns a number to every member of the population, then choses individuals/entities from the population at regular intervals (e.g. every [latex]4[/latex]th individual from a randomly selected starting point).

Stratified sampling divides a population into groups via some criterion, then uses simple random selection or systematic selection to collect a sample from each group.

Convenience sampling selects a sample most accessible to the researcher.

The Main Idea

Non-random sampling methods can exclude certain members of a population from having a chance to be selected in the sample. A sampling method like this is called a biased sampling method because it will tend to favor certain population groups over others.

You learned about different kinds of sampling bias in this section.

• Sampling bias – when the sample is not representative of the population
• Voluntary response bias – the sampling bias that often occurs when the sample is volunteers
• Self-interest study – bias that can occur when the researchers have an interest in the outcome
• Response bias – when the responder gives inaccurate responses for any reason
• Perceived lack of anonymity – when the responder fears giving an honest answer might negatively affect them
• Loaded questions – when the question wording influences the responses
• Non-response bias – when people refusing to participate in the study can influence the validity of the outcome
• Undercoverage occurs when some groups of the population are left out of the sampling process.

The Main Idea

• An observational study is a study based on observations or measurements
• An experiment is a study in which the effects of a treatment are measured
• A treatment in an experiment refers to a specific intervention or condition that is applied to the subjects or participants in order to test its effects on a particular outcome variable.
• The experimental group is the group that receives the treatment
• The control group is the group that does not receive the treatment
• Confounding occurs when there are two potential variables that could have caused the outcome and it is not possible to determine which actually caused the result.
• The placebo effect is when the effectiveness of a treatment is influenced by the patient’s perception of how effective they think the treatment will be, so a result might be seen even if the treatment is ineffectual.
• A placebo is a dummy treatment given to control for the placebo effect.
• An experiment that gives the control group a placebo is called a placebo controlled experiment.
• A blind study is one in which the participant does not know whether they are receiving the treatment or a placebo.
• A double-blind study is one in which those interacting with the participants don’t know who is in the treatment group and who is in the control group.