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Stats Vocabulary

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Answer
Skewed distribution   A distribution that is not symmetrical, but instead has a tail that trails to the right or the left; indicates that the mean, median, and more are not all the same number, but that the majority of the data is not gathered at the mean.  
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Interval scale   A scale in which the values are evenly distributed and arranged in order.  
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Mean   The average value of all of the data; unreliable because it is easily altered by an outlier.  
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Experimental method   Involves controlled variables and observed variables, the manipulation of a variable, and the ultimate conclusion of causation.  
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Dependent variable   The variable that is not manipulated, but that depends on the manipulated variable. This is usually what we are trying to measure.  
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Range   Represents the amount of variability in the data, shown by subtracting the smallest value from the largest value.  
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Inferential statistics   techniques that allow us to study samples and then make generalizations about the populations from which they were selected  
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Positively skewed   when the tail of a distribution points towards the positive end on the x-axis (right-hand side)  
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Median   the score that divides a distribution in half so that 50% of the individuals in a distribution have scores at or below the median  
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Population   the set of all the individuals of interest in a particular study  
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Point-biserial correlation   a special version of the Pearson correlation used to measure the relationship between two variables in situations where one variable is regular, numerical scores but the second only has two values  
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Interquartile range   a range that ignores extreme scores and instead focuses on the range covered by the middle 50% of the distribution  
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Degrees of freedom   the number of scores in a sample that are independent and free to vary; because the sample mean places a restriction on the value of one score in the sample, there are n-1 degrees of freedom for the sample  
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Descriptive statistics   statistical procedures used to summarize, organize, and simplify data  
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Nonparametric test   tests for data that are not arranged in numerical means, but by nominal or ordinal scales instead  
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Residual variance/error variance   the variance that exists in a set of sample data; indicates that the sample variance represents unexplained and uncontrolled differences between scores  
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Normal distribution   a distribution of scores that is symmetrical, in which the mean, median and mode are all the same value  
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Tail   the ends of the distribution of scores that represent the least-occurring scores  
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Percentile rank   a number which represents the place of a single score in relation to the rest of the scores  
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Central limit theorem   For any population with mean (mew) and standard deviation (ro), the distribution of sample means for sample size n will have a mean of (mew) and a standard deviation of (ro)/square root n and will approach a normal distribution as n approaches infinity  
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Correlation   used to measure and describe a relationship between two variables -- without manipulation or control of the variables, and without attempt to justify the relationship  
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Observed frequency   the values that result from counting the number of n individuals in a category (chi-square test)  
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Power   the probability that a test will correctly reject a false null hypothesis -- the probability that the test will identify a treatment effect if one really exists  
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Point estimate   an estimation that uses a single number as an estimate of an unknown quantity; very precise  
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Main effect   the difference between the means in ANOVA  
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Interaction   between two factors -- occurs whenever the mean differences between individual treatment conditions, or cells, are different from what would be predicted from the overall main effects of the factors  
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Parametric test   tests that concern parameters and require assumptions about parameters (t-tests, ANOVA, etc.)  
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Sample   a set of individuals selected from a population, usually intended to represent the population in a research study  
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Sampling error   the discrepancy, or amount of error, that exists between a sample statistic and the corresponding population parameter  
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Correlational method   two different variables are observed to determine whether there is a relationship between them  
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Independent variable   the variable that is manipulated by the researcher  
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Nominal scale   A scale created purely by non-numerical information; categorical scale, like types of cars or authors.  
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Ratio scale   A scale of evenly distributed values, but in which the zero value actually means zero.  
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Symmetrical distribution   a distribution in which it is possible to draw a vertical line through the middle so that one side of the distribution is a mirror image of the other  
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Negatively skewed   in a distribution; when the tail points to the left  
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Mode   the value with the most frequency  
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Variability   the average squared distance from the mean  
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Ordinal scale   A scale arranged by ranks, like the first five runners in a race, in which the order matters.  
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Standard deviation   the square root of the variance  
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Z scores   the precise location of each X value within a distribution; the sign signifies whether the score is above the mean or below the mean; the number specifies the distance from the mean by counting the number of standard deviations  
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Critical region   composed of extreme sample values that are very unlikely to be obtained if the null hypothesis is true; boundaries are determined by the alpha level; null hypothesis is rejected of the data fall in the critical region  
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Interval estimate   in which a range of values is used as an estimate of an unknown quantity  
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Random sample   requires that each individual in the population has an equal chance of being selected; probabilities must stay constant from one selection to the next  
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Body   the larger section of the distribution  
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Binomial distribution   shows the probability associated with each value of X from X=0 to X=n  
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Hypothesis test   a statistical method that uses sample data to evaluate a hypothesis about a population  
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Alternative hypothesis   states that there is a change, a difference, or a relationship for the general population; predicts that the independent variable does have an effect on the dependent variable  
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Type I error   occurs when a researcher rejects a null hypothesis that is actually true; means that the researcher concludes that a treatment does have an effect when in fact it does not  
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Post-hoc tests   additional hypothesis tests that are done after an ANOVA to determine exactly which mean differences are significant and which are not  
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Null hypothesis   states that in the general population there is no change, no difference, or no relationship; predicts that the independent variable has no effect on the dependent variable  
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Type II error   occurs when a researcher fails to reject a null hypothesis that is really false; means that the hypothesis test has failed to detect a real treatment effect  
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