STATS – PSYC 2300
Basic Experimental methods
• Experimental method: one variable is manipulated while another variable is observed
and measured; this attempts to control all other variables from influencing the results
• Manipulation: changing one variable by changing its value from one level to another
• Control: exercising control over the situation to ensure other variables do not influence
the relationship
• Participant variables: characteristics like age, gender, intelligence
• Experimental variable: characteristics of the environment like lighting, time of day, and
weather
• Independent variable: variable that is manipulated by the researcher
• Dependent variable: variable that is observed to assess the effect of the treatment
• Control condition (control group): individuals who receive no treatment or a neutral,
placebo treatment acting as a baseline
• Experimental condition (group): do receive the experimental treatment
• Quasi-independent variable: the “independent” variable in a non-experimental study
Scales of Measurement
• Nominal Scale: set of categories that have different names (label and categorize but
don’t make any quantitative distinctions i.e. gender)
• Ordinal Scale: set of categories organized in an ordered sequence (rank in terms of
size/magnitude i.e. clothing size, Olympic medals)
• Interval scale: ordered categories that are intervals of exactly the same size (zero is
arbitrary i.e. IQ, golf scores)
• Ratio scale: interval scale WITH an absolute zero point (i.e. time to complete a task, # of
correct answers)
Frequency Distributions
• Organized table of the number of individuals located in each category on the scale of
measurement
• Can be a table or a graph
8 9 8 7 10 9 6 4 9 8 7 8 10 9 8 6 9 7 8 9
X f
10 2
9 6
8 6
7 3
6 2
5 0
4 1
Grouped Frequency Distributions
• Should have no more than 10 intervals
• Width of each interval should be simple (2,5,10) • The bottom score in each interval should be a multiple of the width
• All intervals should be the same width
82 75 88 93 53 84 87 58 72 94 69 84 61 91 64 87 84 70 76 89 75 80 73 78 60
X f
90-94 3
85-89 4
80-84 5
75-79 4
70-74 3
65-69 1
60-64 3
55-59 1
50-54 1
Frequency distribution graph
Central Tendency
• Central tendency: statistical measure attempting to determine the single value that is
most representative of the entire set
• Mean: sum of scores divided by the number of scores
∑ X
Population: µ = N
∑ X

More
Less