Chapter 4

experimental design

1. SRS- number the entire population, draw numbers from a hat (every set of n individuals has equal chance of selection 2. stratified- split the population into homogeneous groups, select an SRS from each group 3. cluster- split the population into heterogeneous groups called clusters and randomly select whole clusters for the sample. example: choosing a carton of eggs actually chooses a cluster of eggs

convenience sample

A sample selected by taking the members of the population that are easiest to reach.

experiment or observational study?

A study is an experiment ONLY if researchers IMPOSE a treatment upon the experimental units. In an observational study researchers make no attempt to influence the results.

Census

A study that attempts to collect data from every individual in the population

lurking variable

A variable that is not among the explanatory or response in the study but that may influence the response variable.

cluster sample

All individuals in the chose group are included in the sample.

single-blind

An experiment in which either the subjects or those who interact with them, but not both, know which treatment a subject received.

double-blind

An experiment in which neither the subjects nor those who interact with them know which treatment a subject received.

statistically significant

An observed effect so large that it would rarely occur by chance

Factor

Another name for the explanatory variable in an experiment

sampling techniques

CDR (completely randomized design) - All experimental units are allocated at random among all treatments RBD (randomized block design)- experimental units are put into homogenous blocks. The random assignment of the units to the treatments is carried out separately within each block. Matched Pairs- A form of blocking in which each subject receives both treatments in a random order or the subjects are matched in pairs as closely as possible and one subject in each pair receives each treatment determined at random.

experiment

Deliberately imposes some treatment on individuals to measure their responses.

Control

Done by using a comparative design and ensuring the only systematic difference between the groups is the treatment

subjects

Experimental units that are human beings

Why use a control group?

Gives the researchers a comparison group to be used to evaluate the effectiveness of the treatment(s)

population

In a statistical student, the entire group of individuals about which we want information

experimental units

Individuals to which treatments are applied.

sampling error

Mistakes made in the process of taking a sample. Bad sampling methods and undercoverage are common types.

observational study

Observes individuals and measures variables of interest but does not attempt to influence the responses.

under-coverage

Occurs when some members of the population are left out of the sampling frame

non-sampling error

Some common examples are nonresponse, response bias, and errors due to question wording.

Advantage of using a stratified random sample over an SRS

Stratified random sampling guaranteed that each if the strata will be represented. When strata are chosen properly, a stratified random sample will produce better (less variable/more precise) information that an SRS of the same size

Goal of blocking benefit of blocking

The goal of blocking is to create groups of homogeneous experimental units. The benefit of blocking is the reduction of the effect of variation within the experimental units

cause and effect

Using the results of an experiment to conclude that the treatments caused the difference in responses

anonymity

When the names of individuals participating in a study are not known even to the director of the study

Confounding

When two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other

confidentiality

a basic principle of data ethics that requires individual data to be kept private

matched pair

a common form of blocking for comparing just two treatments

block

a group of experimental units known before the experiment to be similar in some way that is expected to affect responses

margin of error

a numerical estimate of how far the sample result is likely to be from the truth about the population due to sampling variability

level

a specific value of an explanatory variable in an experiment.

explanatory variable

a variable that helps explain of influences changes in the other variable

response variable

a variable that measures the outcome of a study

placebo

an inactive (fake) treatment.

does ______cause _________?

association is NOT causation. an observed association, no matter how strong, is not evidence of causation. Only a well-designed, controlled experiment can lead to conclusions of cause and effect.

placebo effect

describes the fact that some subjects respond favorably to any treatment, even an inactive one.

randomized block design

random assignment of treatments is carried out separately within each block

SRS

sample taken in such a way that every set of n individuals has an equal chance to be the sample actually selected.

bias

the design of a statistical study systematically favors certain outcomes

sample

the part of the population from which we actually collect information

random sampling

the use of chance to select a sample

random assignment

use some chance process to assign experimental units to treatments