Frequency Distribution
organized tabulation of how many people are in each category
- places scores in order
- groups folks with same score together
Σf
how many are in a group
Σx
number times frequency
Proportion
fraction of total group that have the same score p=f/n
- also called relative frequency
Percentage
- fraction as decimal x 100
Grouped frequency distribution
- when there is too much data to list
- create a group of frequency table
-scores grouped into even intervals
Class intervals
what we call each interval
- try to have about 10
- keep widths simple numbers
- all intervals are multiple of width
Percentile rank
percentage of people who score at or below a certain value
- if 70% of the class scores lower than you on the exam
- you are in the 70th percentile
Cumulative Frequency
- accumulation of scores as you move up the scale
- Σf of people greater than any x values
Cumulative Percentage
- % of scores accumulated as you move up the scale
- start at lowest value and tally frequency
- go to next highest value and add previous
- should give you a line the increases as value increases
How to find percentile rank
- use frequency table
- start with lowest class interval
- find f of scores at or below each point
- get the cumulative frequency
- convert cumulative frequency into cumulative %
Normal Curves
- bread and butter of psychology
- commonly occurring population distribution
- greatest frequencies in the middle
- smallest frequencies at extreme
- symmetrical
Skew
the shape of distributions
Central tendency
center of distribution
Variability
how spread out the scores are, wide range vs. clustered together
distributions
three characteristics = shape, central tendency, variability
positive skew
- tail long on the right, most scores cluster on the left, going towards low values
Negative skew
- tail long on the left, more scores cluster on the right
Rules of graph making
- minimize the junk
- plan before you start crating
- say what you mean… mean what you say
- label everything
- communicate ones ideas
- maintain the scale in the graph
- remember simple is best
- limit the number of words
Graphs are powerful tools
- we are visual species
- make an idea simple and easy to understand
- BUT
- our eyes can deceive us
– graphs can be misleading and inaccurate
Good questions to ask
- is information presented correctly?
- does everything look as it should?
- is it clear what they are trying to say?
- is the graph trying to influence you?
- what is the motivation of the graph maker?
