Calculate r (correlation coefficient) and p (probability) value using VassarStats.

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L2 – Correlation


(PSYC-110-B05) Experimental Psychology

Correlation is a statistical measure of the extent to which two variables are

associated or related. Expressed in numerical terms, correlation

coefficients (r) range from -1 to +1.

A positive correlation exists when the two variables increase or decrease

together, in the same direction. e,g, education and income, exercise and

longevity. In contrast, a negative correlation exists when an increase in

one variable is accompanied by a decrease in the other and vice versa. e.g.

optimism and illness, shyness and friendships.

Correlation coefficients vary not only in direction but also in strength. The

higher a correlation is, regardless of whether it is positive or negative, the

stronger the link is between variables. Correlations that are very low, near

zero, indicate that two variables are independent and not related.

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For this lab, come up with two variables that you think will be positively

correlated, and two variables negatively correlated, and the units of

measure. e.g. if amount of sleep is the varible, the units are the hours per

night. For some variables a scale can be used – e.g. for the variable of

tiredness, use a scale of 1 to 10 (1 = not tired to 10 = very tired). Use only

one scaled variable for each pair of variables at the most. This is because

everyone’s idea about tiredness might be different for example, and this

type of data are not as ‘accurate’ as hours of sleep for example.


variable 1: amount of sleep

variable 2: tiredness

units: hours per night units: scale 1-10 (1= not tired, 10= very tired)

Subject #1 7 hrs 6 Subject #1 5 6

2 6 hrs 7 2 7 7

3 8 hrs 8 3 3 8

4 9 4 9

5 10 5 10

*Collect data from 10 participants (real life data preferred instead of

hypothetical data). You can ask each person for answers to all 4 variables Submit Cancel

(2 for the positive, 2 negative). Make sure numbers from variables are

entered as pairs (from same subject).

*Create a scatterplot (use Excel – see lab manual page 18)

*Calculate r (correlation coefficient) and p (probability) value using


r – see lab manual pp.16-19 to find out how to calculate the correlation

coefficient by hand (steps are shown but manual calculation is not

required for this lab). Use Excel and VassarStats to calculate r and p.

p – Probability indicates the degree to which an event is likely to occur

by chance.

In psychological research statistics, if the probability of the outcome (p)

is less than .05 (5%), the result is said to be significant (did not

happen by chance). For this lab, we can make a conclusion that there

is a significant correlation between 2 variables if the calculated p-value is

less than .05.

Read text pages 31-35 to understand the correlational method.

Read lab manual glossary to understand the concepts of probability (p),

level of significance (p< .05), and the meaning of statistical significance

(also in text pages 46-47).

Complete the attached lab sheet and submit.

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Due Date

Sep 16, 2018 11:59 PM


correlation.docx (16.39 KB)

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