StatisticsAP Psychology 2010

J. Mulder

Why are statistics important?

“Proof is virtually impossible for psychology researchers to attain because controlling enough variables to isolate a relationship definitively is, well, virtually impossible.”

-Elliott Hammer Xavier University

of Louisiana

Why are statistics important?

• Assessing Dependability– Absent of “proof”, we have to rely on the

assessment of the likelihood that one’s results are dependable.

– That likelihood never reaches 100%, BUT it can come close.

– Researchers agree that close enough to count is 95%

Statistical Significance

• 95% -> means that 95% of the time what we thinks happen will happen.

• We have only a 5% chance of being wrong in a claim.

• This value is called (alpha)• When psychologists do research they

are trying to find significant results, which means < .05

???Experiment to test a new math program.

Experimental Group= New Program

Control Group = No Program

Results:Experimental Group= 47.5Control Group = 45.6

How do I assess the likelihood that the score from the experimental group would have occurred simply by chance and not because the treatment had an effect?

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Lucky YOU We’re not going to be working very

much with inferential statistics!

BUT you should understand what is meant by statistical significance!

Number ScalesPsychological data can be

collected and measured numerically using one of four scales.

• Nominal• Ordinal• Interval• Ratio

NominalThese numbers are purely for

categorizing data into groups. They have no quantitative properties.

Channels on TV OR

A survey where 1 = Democrat, 2 = Republican, or 3 = Independent

Ordinal

These numbers contain some quantitative information, namely that

of determining ranking (Better, Faster, Smarter). However, The

distance between scale points is not equal.

NCAA Seed Rankings, Positions in a race.

Interval

These numbers contain quantities information. The spaces in between the numbers mean something, and they can be added and subtracted. Interval scales however, have no true zero point.

Temperature (Fahrenheit or Celsius)

Intelligence Test Score

Ratio

These numbers contain the most quantitative information. They can also be added, subtracted, multiplied, and divided, and they have a true zero point.

Money, speed, height, gpa.

Descriptive Statistics

Mode: The most frequently occurring score in a distribution.

Mean: The arithmetic average of scores in a distribution obtained by adding the scores and then dividing by their number.

Median: The middle score in a rank-ordered distribution.

Measures of Central Tendency

Measures of Central Tendency

4, 5, 6, 8, 8, 9, 10, 11,12

Mode = 8

Median = 8

Mean = 8.11

(4+5+6+8+8+9+10+11+12)/9

Measures of Dispersion

Range: a measure of the absolute spread of the scores.

Variance (Standard Deviation): This is a numerical representation of the dispersion of scores around the mean. The smaller the number is, the less spread out the distribution of scores is around the mean.

Activity

Let’s Practice!

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Always RememberTo think Critically!

Meaningful description of data is important in research. Misrepresentation

can lead to incorrect conclusions.