Linear regression models

 

Notes on linear regression analysis (pdf file)

Introduction to linear regression analysis

Mathematics of simple regression

Regression examples

·         Baseball batting averages

·         Beer sales vs. price, part 1: descriptive analysis

·         Beer sales vs. price, part 2: fitting a simple model

·         Beer sales vs. price, part 3: transformations of variables

·         Beer sales vs. price, part 4: additional predictors

·         NC natural gas consumption vs. temperature

What to look for in regression output

What’s a good value for R-squared?
What's the bottom line? How to compare models
Testing the assumptions of linear regression
Additional notes on regression analysis
Stepwise and all-possible-regressions
Excel file with simple regression formulas

Excel file with regression formulas in matrix form

If you are a PC Excel user, you must check this out:
RegressIt: free Excel add-in for linear regression and multivariate data analysis

 

Regression example, part 2:  fitting a simple model


Having already performed some descriptive data analysis in which we learned quite a bit about relationships and time patterns among the beer price and beer sales variables, let’s naively proceed to fit a simple regression model to predict sales of 18-packs from price of 18-packs.  I say “naively” because, although we know that there is a very strong relationship between price and demand, the scatterplot indicated that there is a problem with one of the assumptions of a regression model, namely that vertical deviations from the regression line (prediction errors) ought to have roughly the same variance for small and large predictions.  Also, there are some logical problems with a model that assumes the relation between sales and price to be perfectly linear in situations where the smallest values of sales are tiny in comparison to the largest ones.  Putting those concerns aside for the moment, here is the standard regression summary output (as formatted by RegressIt) for a model in which SALES_18PK is the dependent variable and PRICE_18PK is the independent variable:

The numbers are interpreted as follows:

a.    The estimated regression equation is printed out at the top:  Predicted CASES_18PK = 1,812 - 93.007*PRICE_18PK.  It shows how the coefficients that appear in the regression summary table below are to be used in predicting sales from price.  This is the equation of a straight line whose intercept is 1812 (the height of the line at the point where PRICE_18PK is equal to zero, which it will never come close to in practice) and which has a slope of -93.007, which means the model predicts that 93 fewer cases worth of 18-packs will be sold per $1 increase in the price per case. 

The intercept in a regression model is rarely a number with any direct economic or physical meaning.  The important thing to keep in mind about a regression model is that the regression line always passes through the center of mass of the data, i.e., the point in coordinate space at which all variables are equal to their mean values.  The slope coefficient(s) tell you how the expected value of the dependent variable will move away from its mean value as the independent variables move away from their own mean values.  From the desciptive statistics table, we know that the center of mass of the data for this regression is the point at which price-per-case for 18-packs is $16.73 and the number of cases sold is 257.  Just knowing these two numbers tells us that we should predict 257 cases sold when the price-per-case is $16.73, regardless of the results of fitting the regression model.

b.    The most important  number in the output, besides the model coefficients, is the standard error of the regression, which is 131 in this example.  (To 3 decimal places it is 130.529, but it should be rounded off to an integer value for presentation.  Don’t confuse your audience with more decimal places than are significant.  Many statistical programs routinely display 10 or more digits of precision in their output tables!)   The standard error of the regression is the estimated standard deviation of the “noise” in the dependent variable that is unexplainable by the independent variable(s), and it is a lower bound on the standard deviation of any of the model’s forecast errors, under the assumption that the model is correct.  The estimated standard deviation of a forecast error is actually slightly larger than the standard error of the regression because it must also take into account the errors in estimating the parameters of the model, but there is not much difference if the sample size is reasonably large and the values of the independent variables are not too extreme.  (For details of how the exact standard errors of forecasts are computed, see the  "Mathematics of simple regression" page or “Notes on linear regression” handout.)

From the usual 2-standard-error rule of thumb, it follows that a 95% confidence interval for a forecast from the model is approximately equal to the point forecast plus or minus 2 times the standard error of the regression.  The exact number of standard errors to use in this calculation is the “critical t-value” for a two-tailed 95% confidence interval with 50 degrees of freedom, which is 2.009 as shown at the right end of the regression statistics table.  (The number of “degrees of freedom” for the t-statistic is the number of data points minus the number of coefficients estimated, which is 52 minus 2 in this case.)

So, in this model, the value of 131 for the standard error of the regression means (roughly) that a 95% confidence interval for any forecast will be equal to the point forecast plus-or-minus 262 cases.  We begin to detect a problem here:  the number of cases sold is typically much less than 100 in weeks when the price level is high.  If the model is capable of giving an accurate point forecast in such a week, then it would be silly to compute a lower confidence limit for the forecast by subtracting 262 from it!

c.    Because this is a simple regression model, R-squared is merely the square of the correlation between price and sales:  0.751 = (-0.866)2.   This is the fraction of the variance of the dependent variable that is “explained” by the independent variable, i.e., the fractional amount by which the variance of the errors is less than the variance of the dependent variable.  Adjusted R-squared is an unbiased estimate of the fraction of variable explained, taking into account the sample size and number of variables in the model, and it is always slightly smaller than unadjusted R-squared, although the difference is unimportant in this case:  both numbers round off to 75%.  Adjusted R-squared has a tight connection with the standard error of the regression:  one goes up as the other goes down, for models fitted to the same sample of the same dependent variable.

d.    The estimated intercept is 1812 (cases) with a standard error of 128.  The standard error of a coefficient is the estimated standard deviation of the error in estimating it.  By the usual rule of thumb, an approximate 95% confidence interval for a coefficient is the point estimate plus or minus two standard errors, which is 1812 +/- 2(128) = [1556,2068] for the intercept.  The exact 95% confidence interval is [1555,2069] as shown in the regression summary table.  The difference is trivial, and in any case the confidence limits only have meaning if the model assumptions are approximately correct.

e.    The cofficient of PRICE_18PK is -93 with a standard error of 7.6, and its 95% confidence interval is [-108,-78].   This is not too wide an interval (as these things go), so it appears that we have a reasonably precise estimate of the strength of the price-demand relationship.  Again, though, these numbers are meaningful only if the model assumptions are approximately correct, in particular the assumption that the same price-demand relationship holds across the whole range of prices.

f.     The t-statistic of a coefficient estimate is its point value divided by its standard error, i.e., its “number of standard errors away from zero.”  In general we do not care about the t-stat of the intercept unless it is possible for all the independent variables to simultaneously go to zero and we are interested in whether the prediction should also be zero in such a situation.  (That is not the case here.)  The t-stat of the slope coefficient is -93.007/7.581 = -12.269.   By the usual rule of thumb, a coefficient estimate is significantly different from zero (at the 0.05 level of significance) if its t-stat is greater than 2 in magnitude, which is certainly true here.   (Again, the exact critical t-value is 2.009, not 2, for this model, but the difference is not important.) 

The t-statistic of a coefficient estimate exceeds the specified critical value if and only if the corresponding confidence interval for the coefficient does not include zero:  this is a mathematical identity.  So, it is equivalent to check to see whether the magnitude of the t-statistic exceeds the critical value for the given level of confidence or to check to see whether the correponding confidence interval includes zero.

g.    In the analysis-of-variance table, the only interesting numbers are the F-statistic and its P-value.  The F-statistic tests whether all the independent variables in the model are “jointly” significant, regardless of whether they are individually significant.  In a simple regression model, there is only one independent variable, so the the F-statistic tests its significance alone.  In fact, in a simple regression model, the F-statistic is simply the square of the t-statistic of the slope coefficient, and their P-values are the same.  In this case we have 150.527 = (-12.269)2.  The F-statistic is usually not of interest unless you have a group of variables that should logically be taken together as a unit (say, dummy variables for a set of mutually exclusive conditions, as you might have in a designed experiment), and therefore the analysis-of-variance table is minimized (hidden) by default in RegressIt.

Now, for a better perspective on how well the model fits the data, let’s look at the graphical output.  First, here is the line fit plot, which shows the regression line superimposed on the data.  This is exactly the same line that was superimposed on the scatterplot in the descriptive data analysis, except that it also shows 95% confidence bands for forecasts.



Here we see a couple of problems.  First, as already noted, the unexplained deviations from the regression line are much larger for low price levels than for high price levels.  Under the assumptions of the regression model, they should be approximately the same size across the whole range, as indicated by the fact that the 95% confidence bands have an almost constant width over the whole range.  In theory, at every price level, roughly 95% of the data should fall within the 95% confidence bands, and the vertical deviations from the regression line should be identically normally distributed. Also note that there is a hole in the middle of the price range: no values in between the low $15’s and the low $18’s were observed.  This is not necessarily a problem as far as the regression model is concerned:  there is no requirement that values of the independent variable(s) should have any particular sort of distribution. They might even take on only integer values, perhaps just 0’s and 1’s.  What’s important is how the values of the dependent variable are distributed, for any given values of the independent variable(s).

An even more serious logical problem is also observed in this plot:  the model predicts negative values of sales for prices larger than $19.50 per case (just barely outside the historical range), and the lower 95% confidence limit for a prediction is negative for prices larger than about $16.50 per case.  So, the predictions and confidence intervals for high price levels cannot be taken very seriously!  One alternative modeling strategy that suggests itself is that the sales data for 18-packs could be broken up into two subsets, one for low prices and one for high prices, in light of the fact that intermediate price levels are typically not used.  However, let us suppose that we are interested in predicting what might happen if we were to set the price to some arbitrary value in the middle range, for which it is necessary to fit one model to all the data.

Next let’s look at a time series plot of actual and predicted values, to see how how the forecasts and data lined up on a week-to-week basis.

Here we see that the model slightly overpredicted the moderate sales spikes that occurred early in the year and significantly underpredicted the large spikes that occurred later in the year.  This plot should always be expected to show some regression-to-the-mean on average, but clearly the model is making errors for large predictions in a very systematic way.  In general, this plot is most useful for detecting significant time patterns in the deviations between forecasts and actual values.  Here the problem is not a time pattern in the errors per se.

The remainder of the chart output for this model is shown below.  The residual-vs-observation-number chart  (i.e., residuals versus time, which is always important for time series data) shows some detail that wasn’t quite as apparent on the previous chart, namely that the model made a few serious overpredictions (errors that are negative in sign) along with the serious underpredictions.    This plot is also somewhat unsatisfactory for the fact that it shows that nearly all of the model’s largest errors occurred in the second half of the year.  The reason for this is clear:  most of the price manipulation and most of the spikes in sales occurred in the second half of the year.  Still, the model assumes that the errors should have the same variance at all points in time, regardless of the values of the independent variables.

The residual-vs-predicted plot is where you go to look for evidence of nonlinearities (which would show up here as a curved pattern) or heteroscedasticity (errors that do not have the same variance for all levels of the predictions).  Here we see very strong heteroscedasticity, which was also apparent on the line fit plot:  the model makes bigger errors when making bigger predictions.  For a simple regression model, the residual-vs-predicted plot is just a “tilted” copy of the line fit plot in which the regression line is superimposed on the X-axis, and it is also flipped left-to-right if the slope coefficient is negative, as it is here.

Lastly (at the bottom of this page), we have the residual histogram plot and normal quantile plot which reveal the shape of the error distribution, along with the Anderson-Darling statistic which provides a numerical measure of the degree to which the error distribution is non-normal.   The importance of testing for normality of the error distribution is that the formulas for calculating confidence intervals for forecasts are based on properties of the normal distribution.  If the error distribution is far from normal, then there will be some systematic over- or under-estimation of “tail area probabilities”.   Here, the outliers that are seen in the histogram plot and the S-shaped pattern that is seen in the normal quantile plot indicate a signficantly non-normal distribution, as does the A-D stat, whose value of 2.591 is far above the 0.75 [1.04] critical value for significance at the 0.05 [0.01] level. 

The A-D stat, like other statistical tests for normality, should not be given an exaggerated importance, particularly when sample sizes are very small or very large.  With very small samples it is hard to determine much at all about the shape of the error distribution and with very large samples even a tiny departure from normality will show up as statistically significant.  (My own preference is to focus on shape of the pattern, if any, that is seen on the normal quantile plot, and also to look closely at the most extreme errors to try to figure out what went wrong there.)  And like other diagnostic test statistics for the model assumptions, the A-D stat is not the bottom line, just a little red flag that may or may not wave.   Still, it provides some sort of numerical benchmark to go along with the visual impressions of non-normality that are obtained from the histogram and quantile plots.

What’s the bottom line?  Despite its high R-squared value, this is not a good model.  Its forecasts and their confidence limits are illogical at high price levels.   The relationship between beer sales and beer price is evidently not linear over such a wide range in prices, and the model also badly violates the homoscedasticity and normal-distribution assumptions for the errors.   Let this be a warning:  not all significant relationships are linear, not all random variables are normally distributed, and not all models with high R-squared values are good ones.

Go on to next step:  variable transformations