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Descriptive Statistics

Statistical procedures that organize and summarize, and describe a set of research data.

Frequency Distribution

A distribution of observed frequencies of occurrence of the values of a variable; indicates how often a particular score or observation occurs.

Frequency Polygons

Line graphs that displays frequency distributions

Central Tendency

Mark the center of the distribution. Include mean, median and mode.

Mean

Average of all the scores in the distribution. Most commonly used measure of central tendency, although accuracy can be distorted by extreme scores.

Median

Central score of the distribution.

Mode

Score that appears most frequently.

Outliers (Extreme Scores)

Extremely high or low values in a set of data that can affect the mean and skew distributions.

Positively Skewed

When a distribution includes extreme scores that are very high. Contain more low scores than high scores; the skew is produced by the unusually high score. Curve at left, tail on right.

Negatively Skewed

When a distribution includes extreme scores that are unusually low and the low scores skew the distribution. Contain a higher number of high scores than low scores. Curve at right, tail on left.

Range

Distance between the highest and lowest score in a distribution

Variance

Measures the degree of dispersion or scatter. Standard deviation squared.

Standard Deviation

Measures average distance of any score in the distribution from the mean. The higher it is, the more spread out the distribution.

Z-Scores

Measure the distance of a score from the mean in units of standard deviation. Scores below the mean are negative and scores above the mean are positive.

Normal Curve (bell curve)

Frequency distribution in which most measurements are centered around the mean and the two sides of the distribution are symmetrical. Theoretical bell-shaped curve for which the area under the curve lying between any two z-scores has been predetermined. 68% of scores fall within one standard deviation, 95% within two standard deviations. and 99% within three.

Correlation

Measures the relationship between two variables. Positive=presence of one predicts the presence of the other. Negative=presence of one predicts absence of the other. No relationship=none exists.

Correlation Coefficient

Measures the strength of a correlation. -1 is a perfect negative and +1 is a perfect positive. 0 denotes weakest.

Scatter Plot

Graphs pair of values. Useful for correlations.

Line of Best Fit (Regression Line)

Line drawn through the scatter plot that minimizes the distance of all the points from the line. Upward slope=positive correlation. Downward slope=negative.

Inferential Statistics

Determines whether or not the findings of the study can be applied to the larger population.

Sample Error

The extent to which the sample differs from the population

P Value

The probability level which is used to decide if results are statistically significant (not due to chance); The smaller the value, the more statistically significant.

Statistically Significant Results

A p-value of .05, meaning there is a 5% chance of the results occurring by chance.