Biostats Exam 3 (Correlation)

Correlation

a relationship between two variables

Positive relationships

an increase in one variable predicts an increase in the other

Negative relationships

an increase in one variable predicts a decrease in the other

A correlation alone cannot be used to make a definitive statement about

causation

Sign in a correction

Direction

Number in a correction

Strength

Which graph is the most effective way of presenting relationship data

Scatterplots

Negative relationship scatterplot

slanting downward

Positive relationship scatterplot

Slanting upward

Curvilinear relationships scatterplot

Curve in the graph

Assumptions of the Pearson Correlation

- Uses two variables

- Both quantitative*

- Linear relationship

- Minimal skew/no large outliers

- Must observe the whole range for each variable

Pearson r correlation coefficient

a way of numerically expressing correlation

Pearson r range

-1 to +1

Nonparametric Analysis Spearman's Rank data

for ordinal and skewed data

Kendall's tau-b

for ordinal and skewed data, less affected by error

ETA

a special coefficient used for curvilinear relationships

Interpreting Correlation Values calculation

r^2 * 100

r^2 * 100 meaning

% change in variable accounted by another variable