CHAPTER 13: STATISTICS

• Alternative Hypothesis (H1)


  A statement that the data came from different populations; the research hypothesis, which cannot be tested directly
• Critical Region


  Portion in the tail(s) of the distribution of a test statistic extreme enough to satisfy the researcher's criterion for rejecting the null hypothesis—for instance, the most extreme 5% of a distribution where p < 0.5 is the chosen significance level
• Descriptive Statistics


  The standard procedures used to summarize and describe data quickly and clearly; summary statistics reported for an experiment, including mean, range, and standard deviation
• Experimental Error


  Variation in subjects' scores produced by uncontrolled extraneous variables in the experimental procedure, experimenter bias, or other influences on subjects not related to effects of the independent variable
• Inferential Statistics


  Statistics that can be used as indicators of what is going on in a population; also called test statistics
• Mean


  An arithmetical average computed by dividing the sum of a group of scores by the total number of scores; a measure of central tendency
• Measures of Central Tendency


  Summary statistics that describe what is typical of a distribution of scores; include mean, median, and mode
• Median


  The score that divides a distribution in half, so that half the scores in the distribution fall above the median, half below; a measure of central tendency
• Mode


  The most frequently occurring score in a distribution; a measure of central tendency
• Nondirectional Hypothesis


  A statement that predicts a difference between treatment groups without predicting the exact pattern of results
• Normal Curve


  The distribution of data in a symmetrical, bell-shaped curve
• Null Hypothesis (H0)


  A statement that the performance of treatment groups is so similar that the groups must belong to the same population; a way of saying that the experimental manipulation had no important effect
• One-Tailed Test


  A statistical procedure used when a directional prediction has been made; the critical region of the distribution of the test statistic is measured in just one tail of the distribution
• Range


  The difference between the largest and smallest scores in a set of data; a rough indication of the amount of variability in the data
• Raw Data


  Data recorded as an experiment is run; the responses of individual subjects
• Significance Level


  The statistical criterion for deciding whether to reject the null hypothesis or not, typically p < 0.5
• Standard Deviation


  The square root of the variance; measures the average deviation of scores about the mean, thus reflecting the amount of variability in the data
• Statistical Inference


  A statement made about a population and all its samples based on the samples observed
• Statistical Significance


  Meeting the set criterion for significance; the data do not support the null hypothesis, confirming a difference between the groups that occurred as a result of the experiment
• Statistics


  Quantitative measurements of samples; quantitative data
• Summary Data


  Descriptive statistics computed from the raw data of an experiment, including the measures of central tendency and variability

  
• Test Statistics


  Statistics that can be used as indicators of what is going on in a population and can be used to evaluate results; also called inferential statistics
• Two-Tailed Test


  A statistical procedure used when a nondirectional prediction has been made; the critical region of the distribution of the test statistic is divided over both tails of the distribution
• Type 1 Error


  An error made by rejecting the null hypothesis even though it is really true; stating that an effect exists when it really does not
• Type 2 Error


  An error made by failing to reject the null hypothesis even though it is really false; failing to detect a treatment effect
• Variability


  Fluctuation in data; can be defined numerically as the range, variance, or standard deviation
• Variance


  The average squared deviation of scores from their mean; a more precise measure of variability than the range
• Descriptive statistics describe sample central tendency and variability
• Inferential statistics allow us to draw conclusions about a parent population from a sample
• Just as Detective Katz can at best show that Ms. Adams is probably guilty, in statistics we can only state that the independent variable probably affected the dependent variable
• While we cannot prove that the independent variable definitely caused the change in the dependent variable, we can state probability that our conclusion is correct
• Population


  A set of people, animals, or objects that share at least one characteristic in common (like college sophomores)
• Sample


  A subset of the population that we use to draw inferences about the population
• Statistical inference


  The process by which we make statements about a parent population based on a sample
• The differences in scores obtained from separate treatment groups are not significantly greater than what we might expect between any samples randomly drawn from this population
• When researchers report this outcome, it means that were was not treatment effect
• For a set of dependent variable measurements, there is variability when the scores are different
• Variability "spread out" a sample of scores drawn from a population
• The null hypothesis (H0) is the statement that the scores came from the same population and the independent variable did not significantly affect the dependent variable
• Alternative hypothesis (H1)


  The statement that the scores came from different populations the independent variable significantly affected the dependent variable