notes/education/statistics/Correlation and Regression.md

(Chapter 8, STAT 1040)

Correlation

Scatter Diagrams

A scatter diagram or scatter plot shows the relationship between two variables. One variable is on the X axis, the other on the Y axis.

If a scatter diagram is football shaped, it can be summarized using the 5-number summary:

Variable	Description
\bar{x}	The average of the set graphed along the X axis
\sigma_x	The standard deviation of set graphed along the X axis
\bar{y}	The average of the set graphed along the Y axis
\sigma_y	The standard deviation of the set graphed along the Y axis
r	The correlation coefficient, or how closely clustered the datapoints are in a line

The intersection of the averages of x and y will be the center of an oval shaped scatter diagram. Draw lines 2\sigma (will contain ~95% of all data) from the center along each axis to generalize the shape of a scatter plot.

You can approximate the mean by trying to find the upper bound and the lower bound of 2\sigma deviation to either side of the mean, then finding the middle of those two points to find \bar{x}. You can divide the range between the two points by 4 to find \sigma.

Association

• Positive association is demonstrated when the dots are trend upward as x increases (r is positive).
• Negative association is demonstrated when the the dots trend downward as x increases (r is negative).
• Strong association is demonstrated when dots are clustered tightly together along a line (|r| is closer to 1).
• Weak association is demonstrated when dots are not clustered tightly. (|r| is closer to 0)

Correlation

Correlation is between -1 and 1. Correlation near 1 means tight clustering, and correlation near 0 means loose clustering. r is -1 if the points are on a line with negative slope, r is positive 1 if the points are on a line with a positive slope. As |r| gets closer to 1, the line points cluster more tightly around a line.

If x is above average, we expect the y to be above average if there's a strong correlation coefficient

Calculating r by hand

Put the x values into L1, put the y values into L2.

1. Convert the x each x value in the list to standard units(z). Convert each y value to standard units. This will create two new tables containing z_x and z_y.

 z = \frac{x-\bar{x}}{\sigma_x} 

2. Multiply the standard units for each (z_x, z_y) pair in the sets, giving you a fifth list, named p in this example.

 x * y = p

3. Find the average of the values from step 3, this is r.

 \bar{x}(p) 

https://www.thoughtco.com/how-to-calculate-the-correlation-coefficient-3126228

Terminology

Term	Definition
r	Correlation Coefficient
Linear Correlation	Measures the strength of a line

(Chapter 9, STAT 1040)

Correlation contd.

Notes

• r is a pure number, and does not have units
• r does not change if you:
  • switch the x and y values
  • add the same number to every x or y value
  • multiply each x or y value by a positive number
• The correlation coefficient can be misleading in the presence of outliers or nonlinear association.
• r only shows the strength of a linear relationship.
• High r only indicates high correlation, not causation.
• r is not a linear, perfect scale. An r of 0.8 does not mean twice as much linearity as an r of 0.4

Ecological Correlations

• Sometimes each point on the plot represents the average or rate for a whole group of individuals.
• Ecological correlations are artificially strong
• The size of points may indicate the size of the datapoints

Regression

(Chapter 10, STAT 1040)

Notes

Explanatory and Response Variables

• Regression uses values of one variable to predict values for a related value.
• The variable you are trying to predict is called the response variable. It is graphed along the y-axis. This is the thing being predicted/measured.
• The variable you have information about that you are using to make the prediction is called the explanatory variable. It is graphed along the x-axis. This is the treatment.
• Just because a relationship exists between x and y does not mean that changes in x cause changes in y.
• If the graph is given to you already set up, you already know the response and explanatory variables.
• The \sigma line will always always have a slope of:

\pm \frac{\sigma_y}{\sigma_x}

• The SD line always passes through the averages for each axis.
• It'll go through the middle of the "football"
• (ave_x, ave_y) is on the line
• Visually looks like a line of best fit
• The SD line is not used for prediction because it over-predicts
• Someone who is exactly on the SD line is the same number of SDs above or below the average in the y axis as they are in the x axis. Given a scatter diagram where the average of each set lies on the point (75, 70), with a \sigma_x of 10 and a \sigma_y of 12, you can graph the SD line by going up \sigma_y and right \sigma_x, then connecting that point (in this example, (85, 82)) with the mean points.

The Regression Line/Least Squared Regression Line (LSRL)

• This line has a more moderate slope than the SD line. it does not go through the peaks of the "football"
• Predictions can only be made if the data displays a linear association (is a football shape).
• The regression line is used to predict the y variable when the x variable is given
• In regression, the x variable is the known variable, and y is the value being solved for.
• The regression line goes through the point of averages, and can be positive or negative

 slope = r(\frac{\sigma_y}{\sigma_x}) 

• You can find the regression line by multiplying \sigma_y by r, for the rise, then using \sigma_x for the run from the point of averages.

The below formula can be used to predict a y value given a 5 number summary of a set.

 \hat{y} = \frac{x-\bar{x}}{\sigma_x} * r * \sigma_y + \bar{y} 

1. Find z_x
2. Multiply z_x by r
3. Multiply that by \sigma_y
4. Add the average of y

• For a positive association, for every \sigma_x above average we are in x, the line predicts y to be \sigma_y standard deviations above y.x
• There are two separate regression lines, one for predicting y from x, and one for predicting x from y
• Do not extrapolate outside of the graph

(Ch 12, stat 1040) Predicting a y value for a given x value can be calculated when given the regression equation.

 y = mx + b 

Where y is the predicted value, m is the slope, x is the given value and the b is the intercept.

 slope = \frac{r * \sigma_y}{\sigma_x} 

Residual plots

A plot of the differences from the line. Makes it easier to see if the data is football shaped. If a residual plot has a strong pattern, it may not be suitable for making predictions.

The Regression Effect

• In a test-retest situation, people with low scores tend to improve, and people with high scores tend to do worse. This means that individuals score closer to the average as they retest.
• The regression fallacy is contributing this to something other than chance error.

R.M.S Error for Regression

The distance of an individual point from the regression line. This only applies for a football shaped scatter diagram.

• If a point is below the line, the error is negative.
• If a point is above the line, the error is positive.
• "Give or take"
• residual = observed - predicted for a given x value
• The r.m.s error is the r.m.s size of the errors

 \sqrt{1-r^2}(\sigma_y) 

• On a least squared regression line, the 1 r.m.s error away will contain 2\sigma of the data, and it should loosely mirror a normal curve.

• To approximate the R.M.S error for a scatter diagram, take a high value and a low value for a given x coordinate, and divide by 4, because r.m.s error is within 2\sigma of either side of the line.

• 68% = 2\sigma, 95% = 4\sigma

• RMS can help determine which observations are outliers. Typically if a value is more than 2 r.m.s away from the prediction estimate, it is considered to be an outlier

• The RMS error is only appropriate for homoscedastic scatter diagrams (football shape)

• Heteroscedastic scatter diagrams should not be used to make a prediction, because they do not follow a football shape

• Homoscedastic scatter diagrams can be used to make predictions, because they follow a football shape

---

Terminology

Term	Definition
\hat{y}	The predicted value
Homoscedastic	The scatter diagram will look the same above and below the LSRL
Heteroscedastic	Will have more variability on one side of the regression line