Psychological Discovery 5
Descriptive statistics. Methods that help researchers organise, summarise and
simplify the results obtained from studies.
Inferential statistics. Methods that use the results obtained from samples to help
make generalisations about populations.
Statistic. A summary value that describes a sample e.g. average score.
Parameter. Summary value that describes a population.
Central tendency. Statistical measure that identifies a single store that defines
the centre of the distribution.
Standard deviation. Measure of variability.
Variability. Measure of a spread of scores in a distribution.
Variance. Measures variability by computing the average squared
distance from the mean.
Normal curve. Symmetrical, bell-shaped frequency polygon representing a
Normal distribution. Theoretical frequency distribution that has certain special
Kurtosis. How flat/peaked a normal distribution is.
Mesokurtic. Normal curves that have peaks of medium height and
distributions that are moderate in breadth.
Leptokurtic. Normal curves that are tall and thin, with only a few scores
in the middle of the distribution having a high frequency.
Platykurtic. Normal curves that are short and dispersed.
Positively skewed Peak to the left and tail to the right.
Negatively skewed Peak to the right and tail to the left.
z-score. Measure of how many standard deviation units an
individual’s raw score falls from the mean of the distribution.
Standard normal Mean of zero and standard deviation of one.
Probability. Expected relative frequency of a particular outcome.
Percentile rank. Score that indicates the percentage of people who scored at
or below a given raw score.
Summarising data using descriptive statistics
Role of statistics in the research process
o Help organise and summarise the data so the researcher can see what
happened in the study and communicate to others.
o Help the researcher answer the genera