chapter 8/9, 13/14(one sample t-test), independent t-test

population

Any complete set of observations (or potential observations)

sample

Any subset of observations from a population

random sampling

A selection process that guarantees all potential observations in the population have an equal chance of being selected

• occurs in well-designed surveys

random assignment

A procedure designed to ensure that each subject has an equal chance of being assigned to any group in an experiment

•  occurs in well-designed experiments

probability

The proportion or fraction of times that a particular event is likely to occur.

addition rule

Add together the separate probabilities of several mutually exclusive events to find the probability that any one of these events will occur

multiplication rule

Multiply together the separate probabilities of several independent events to find the probability that these events will occur together

independent events

the occurrence of one event has no effect on the probability that the other event will occur.

gambler’s fallacy

belief that past independent events influence future, independent event (ex: coin flips)

conditional probability

The probability of one event, given the occurrence of another event

“given that”

will be the denominator and the other number will be the numerator

z-score

A unit-free, standardized score that indicates how many standard deviations a score is above or below the mean of its distribution.

z-test

A hypothesis test that evaluates how far the observed sample mean deviates from the population mean

• you have information regarding the population like a mean and standard deviation

one sample t-test

used to determine whether an unknown population mean is different from a specific value

• you don’t have the mean, but you have a hypothesis

standard deviation

 rough measure of the average (or standard) amount by which scores deviate on either side of their mean

standard error of the mean (SEM)

A rough measure of the average amount by which sample means deviate from the population mean

• measures how far the sample mean (average) of the data is likely to be from the true population mean

• will always be smaller than the SD

central limit theorem

Regardless of the population shape, the shape of the sampling distribution of the mean approximates a normal curve if the sample size is sufficiently large

confidence intervals

A range of values that, with a known degree of certainty, includes an unknown population characteristic, such as a population mean

• a range of values we are fairly sure our true value lies in

standardized effect estimate , Cohen’s d

• Describes effect size by expressing the observed mean difference in standard deviation units.

• Report as a positive number - With this number we are concerned with magnitude not direction

one sample assumptions

• that there are no outliers

• that its a roughly normal distribution

  • Shapiro–Wilk test

  • Not discussed in text

  • Not a default in SPSS

Shapiro-wilk test

statistical hypothesis test to determine if a sample of data likely comes from a normally distributed population

• It gives you a p-value

  • If p > 0.05 → the data is approximately normal.

  • If p < 0.05 (significant) → the data is not normal.

independent samples t test

compares the means of two independent groups in order to determine whether there is statistical evidence that the associated population means are significantly different

assumptions of the independent samples t test

1. The data are approximately normally distributed

2. The two samples are independent

3. Equal variances across groups (Levene’s Test in SPSS helps check this)

meta-analysis

A set of data-collecting and statistical procedures designed to summarize the various effects reported by groups of similar studies

• Usually an aggregation of effect sizes such as Cohen’s d

file drawer effect

A false positive caused by an inflated type I error attributable to reports of nonsignificant findings being tossed in a file drawer or waste basket