# Meaning of Important terms in Psychology Part 1 for IAS

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**Descriptive statistics:** Mathematical tools used to describe and summarize numeric data.

**Inferential statistics:** Mathematical tools used for decision making, for generalizing from small samples, and for drawing conclusions.

**Graphical statics:** Techniques for presenting numbers pictorially, often by plotting them on a graph.

**Frequency distribution:** A table that divides an entire range of scores into a series of classes and then records the number of scores that fall into each class.

**Histogram:** A graph of a frequency distribution in which the number of scores falling in each class is represented by vertical bars.

**Frequency polygon:** A graph of a frequency distribution in which the number of scores falling in each class is represented by points on a line.

**Central tendency:** The tendency for a majority of scores to fall in the midrange of possible values.

**Mean:** A measure of central tendency calculated by adding a group of scores and then dividing by the total number of scores.

**Median:** A measure of central tendency found by arranging scores from the highest to the lowest and selecting the score that falls in the middle. That is, half the vales in a group of scores fall above the median and half fall below.

**Mode:** A measure of central tendency found by identifying the most frequently occurring score in a group of scores.

**Variability:** The tendency for a group of scores to differ in value. Measures of variability indicate the degree to which a group of scores differ from one another.

**Range:** The difference between the highest and lowest scores in a group of scores.

**Standard deviation:** An index of how much a typical score differs from the mean of a group of scores.

**Z-score:** A number that tells how many standard deviations above or below the mean a score is.

**Normal curve:** A bell-shaped distribution, with a large number of scores in the middle, tapering to very few extremely high and low scores.

**Correlation:** The existence of a consistent, systematic relationship between two events, measures, or variables.

**Scatter diagram:** A graph that plots the intersection of paired measures; that is, the points at which paired X and Y measures cross.

**Positive relationship:** A mathematical relationship in which increase in one measure are matched by increase in the other (or decreases correspond with decreases).

**Zero correlation:** The absence of a (linear) mathematical relationship between two measures.

**Negative relationship:** A mathematical relationship in which increases in one measure are matched by decreases in the other.

**Coefficient of correlation:** A statistical index ranging from -1.00 to +1.00 that indicates the direction and degree of correlation.

**Perfect positive relationship:** A mathematical relationship in which the correlation between two measures is +1.00.

**Perfect negative relationship:** A mathematical relationship in which the correlation between two measures is -1.00.

**Percent of variance:** A portion of the total amount of variation in a group of scores.

**Population:** An entire group of animals, people, or object belonging to a particular category (for example, all college students or all married women).

**Sample:** A smaller subpart of a population.

**Representative sample:** a small, randomly selected part of a larger population that accurately reflects characteristics of the whole population.

**Random selection:** Choosing a sample so that each member of the population has an equal chance of being included in the sample.

**Statistical significance:** The degree to which an event (such as the results of an experiment) is unlikely to have occurred by chance alone.