Experiments Design

#statistics #experimentaldesign #sampling #randomvariable

Experimental Study

A researcher conducts an experimental study, or more simply, an experiment, by assigning subjects to certain experimental conditions and then observing outcomes on the response variable (or variables). The experimental conditions, which correspond to assigned values of the explanatory variable, are called treatments.

Observational Study

In an observational study, the researcher observes values of the response variable and explanatory variables for the sampled subjects, without anything being done to the subjects (such as imposing a treatment)

Sample Survey

A sample survey selects a sample of subjects from a population and collects data from them. In the field of statistics a survey does not just indicate an opinion poll or a questionnaire; it includes any information gathered from or about the subject

Sampling Frame

The sampling frame is the list of subjects in the population from which the sample is taken.

Simple Random Sample

A simple random sample of n subjects from a population is one in which each possible sample of that size has the same chance of being selected.

Types of Bias in Sample Surveys

1. Sampling bias occurs from using nonrandom samples or having under coverage.
2. Nonresponse bias occurs when some sampled subjects cannot be reached or refuse to participate or fail to answer some questions.
3. Response bias occurs when the subject gives an incorrect response (perhaps

lying) or the way the interviewer asks the questions (or wording of a question in print) is confusing or misleading.

The Role of Randomization in Experiments

Use randomization for assigning subjects to the treatments

1. To eliminate bias that may result if you (the researchers) assign the subjects
2. To balance the groups on variables that you know affect the response
3. To balance the groups on lurking variables that may be unknown to you

Key Parts of a Good Experiment

1. A good experiment has a control comparison group, randomization in assigning experimental units to treatments, and blinding.
2. The experimental units are the subjects—the people, animals, or other objects to which the treatments are applied
3. The treatments are the experimental conditions imposed on the experimental units. One of these may be a control (for instance, either a placebo or an existing treatment) that provides a basis for determining whether a particular treatment is effective. The treatments correspond to values of an explanatory variable.
4. Randomly assign the experimental units to the treatments. This tends to balance the comparison groups with respect to lurking variables
5. Replication of studies increases confidence in the conclusions.