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### Population parameteres

specific characteristics of the population that a statistical study is designed to estimate.

### representative sample

is a sample in which the relevant characteristics of the sample members match those of the population.

### simple random sampling

we choose a sample of items in such a way that every sample of a given size has an equal chance of being selected

### systematic sampling

we use a simple system to choose the sample, such as selecting every 10th or every 50th member of the population.

### convenience sampling

we use a sample that is convenient to select, such as people who happen to be in the same classroom

### stratified sampling

we use this method when we ar concerned about differences among subgroups, or strata, within a population. we first identify the subgroups and then draw a simple random sample with in each subgroup. the total sample consists of all the samples from the individual subgroups.

### observational study

researchers observe or measure characteristics of the sample members but do not attempt to influence or modify these characteristics

### experiment

researchers apply a treatment to some or all of the sample members and then look to see whether the treatment has any effects.

### treatment group

in an experiment is the group of sample members who receive the treatment being tested.

### placebo

lacks the active ingredient of a treatment being tested in a study, but is identical in appearance to the treatment.

### placebo effect

refers to the sistuation in which patients imporve simply b/c they believe they are receiving a useful treatment.

### single blind

if the participants do not know whether thay are members of the treatment group or members of the control group , but the experiments do know.

### double blind

participants and experimenters do not know who belongs to the treatment group or control group.

### margin of error

is used to describe a confidence interval that is likely to contain the tru population paramenter. We find this interval by subtracting and adding the margin of error from the sampl statistic obtained in the study. ie.from = sample stat - margin of error or to = sample stat + margin of error

### selection bias

occurs whenever researchers select their sample in a way that tends to make it unrepresentative of the population.

### variable

any item or quantity that can vary or take on different values. In statistics it is the items you seek to measure.

### histogram

is a bar graph for quantitative data catefories. the bars have natural order and the bar widths have specific meaning.

### correlation

exists between two variables when higher values of one variable consistently go with higher values of another or when higher values of one variable consistently go wieht lower values of another.