Regression

Misc

  • Collinearity - Umbrella term encompassing any linear association between variables.
  • Multicollinearity - Specific type of collinearity where the linear relationship is particularly strong and involves multiple variables.

GOF

{performance}

  • Handles all kinds of models including mixed models, bayesian models, econometric models
  • performance::check_model
    • Can take a tidymodel as input.
  • performance::model_performance
    • Scores model using AIC, AICc, BIC, R2, R2adj, RMSE, Sigma
      • “Sigma” is the standard deviation of the residuals (aka Residual standard error, see below)
  • performance::compare_performance
    • Outputs table with scores for each model

summary

  • Standard Errors: An estimate of how much estimates would ‘bounce around’ from sample to sample, if we were to repeat this process over and over and over again.

    • More specifically, it is an estimate of the standard deviation of the sampling distribution of the estimate.

  • t-score: Ratio of parameter estimate and its SE

    • Used for hypothesis testing, specifically to test whether the parameter estimate is ‘significantly’ different from 0.

  • p-value: The probability of finding an estimated value that is as far or further from 0, if the null hypothesis were true.

    • Note that if the null hypothesis is not true, it is not clear that this value is telling us anything meaningful at all.

  • F-statistic: This “simultaneous” test checks to see if the model as a whole predicts the response variable better than that of the intercept only (a.k.a. mean) model.

    • i.e. Whether or not all the coefficient estimates should jointly be considered unable to be differentiated from 0
    • Assumes homoskedastic variance
    • If this statistic’s p-value < 0.05, then this suggests that at least some of the parameter estimates are not equal to 0
    • Many authors recommend ignoring the P values for individual regression coefficients if the overall F ratio is not statistically significant. This is because of the multiple testing problem. In other words, your p-value and f-value should both be statistically significant in order to correctly interpret the results.
    • Unless you only have an intercept model, you have multiple tests (e.g. variables + intercept p-values). There is no protection from the problem of multiple comparisons without this.
      • Bear in mind that because p-values are random variables–whether something is significant would vary from experiment to experiment, if the experiment were re-run–it is possible for these to be inconsistent with each other.

  • Residual standard error - Variation around the regression line.

    \[ \text{Residual Standard Error} = \sqrt{\frac{\sum_{i=1}^n (Y_i-\hat Y_i)^2}{dof}} \]

    • \(\text{dof} = n − k^*\) is the model’s degrees of freedom
      • \(k^*\) = The numbers of parameters you’re estimating including the intercept
    • Aside from model comparison, can also be compared to the sd of the observed outcome variable

Kolmogorov–Smirnov test (KS)

  • Guessing this can be used for GOF to compare predictions to observed
  • Misc
    • See Distributions >> Tests for more details
    • Vectors may need to be standardized (e.g. normality test) first unless comparing two samples
  • Packages
    • {KSgeneral} has tests to use for contiuous, mixed, and discrete distributions; written in C++
    • {stats} and {dgof} also have functions, ks.test
    • All functions take a numeric vector and a base R density function (e.g. pnorm, pexp, etc.) as args
    • {KSgeneral} docs don’t say you can supply your own comparison sample (2nd arg) only the density function but with stats and dgof, you can.
      • Although they have function to compute the CDFs, so if you need speed, it might be possible to use their functions and do it man

g-index

  • From Harrell’s rms (doc)
  • I think the g-index for a model is the total of all the partial g-indexes
    • Each independent variable would have a partial g-index
    • He also supplies 3 different ways of combining the partial indexes I think
    • Harrell has a pretty thorough example in the function doc that might shed light
  • Partial g-Index
    • Example: A regression model having independent variables, age + sex + age*sex, with corresponding regression coefficients \(\beta_1\), \(\beta_2\), \(\beta_3\)
      • g-indexage = Gini’s mean difference (age * (\(\beta_1\) + \(\beta_3\)*w))
        • Where w is an indicator set to one for observations with sex not equal to the reference value.
        • When there are nonlinear terms associated with a predictor, these terms will also be combined.

Residuals

  • Misc

    • Packages
      • {gglm} - Diagnostic plots for residuals
    • influence.measures(mod) wll calculate DFBETAS for each model variable, DFFITS, covariance ratios, Cook’s distances and the diagonal elements of the hat matrix. Cases which are influential with respect to any of these measures are marked with an asterisk.

  • Standardized Residuals:

    \[ R_i = \frac{r_i}{SD(r)} \]

    • Where r is the residuals
    • Follows a Chi-Square distribution
    • Any \(|R|\) > 2 or 3 is an indication that the point may be an outlier
    • In R: rstandard(mod)
    • It may be a good idea to run the regression twice — with and without the outliers to see how much they have an effect on the results.
    • Inflation of the MSE due to outliers will affect the width of CIs and PIs but not predicted values, hypothesis test results, or effect point estimates

  • Studentized Residuals

    \[ t_i = r_i \left(\frac{n-k-2}{n-k-1-r_i^2}\right)^{\frac{1}{2}} \]

    • Where \(r_i\) is the ith standardized residual, \(n\) = the number of observations, and \(k\) = the number of predictors.
    • Some outliers won’t be flagged as outlierrs because they drag the regression line towards them. These residuals are for detecting them.
      • Therefore, generally better at detecting outliers than standardized residuals.
    • The studentized residuals, \(t\), follow a student t distribution with dof = n–k–2
      • Rule of thumb is any \(|t_i|\) > 3 is considered an outlier but you can check the residual against the critical value to be sure.
    • In R, rstudent(mod)
    • It may be a good idea to run the regression twice — with and without the outliers to see how much they have an effect on the results.
    • Inflation of the MSE due to outliers will affect the width of CIs and PIs but not predicted values, hypothesis test results, or effect point estimates

  • Check for Normality

    • Residuals vs Fitted scatter
      • Looking for data to centered around 0
      • Helpful to have a horizontal and vertical line at the zero markers on the X & Y axis.
    • Residuals historgram
      • Look for symmetry
      • Helpful to have a gaussian overlay

  • Check for heteroskedacity or Non-Linear Patterns

    • Residuals vs Fitted scatter

      • Heteroskedastic

        • Subtler but present (reverse megaphone shape)

        • Solutions:

      • Nonlinear

      • Breusch Pagan test (lmtest::bptest or car::ncvtest )

        • H0: No heteroskedacity present
        • bptest takes data + formula or lm model; ncvtest only takes a lm model

  • Check for Autocorrelation

    • Run Durbin-Watson, Breusch-Godfrey tests: forecast::checkresiduals(fit)  to look for autocorrelation between residuals.
      • Range: 1.5 to 2.5
      • Close to 2 which means you’re in the clear.

  • Check for potential variable transformations

    • Residual vs Predictor
      • Run every predictor in the model and every predictor that wasn’t used.
      • Should look random.
      • Nonlinear patterns suggest non linear model should be used that variable (square, splines, gam, etc.). Linear patterns in predictors that weren’t use suggest they should be used.
    • car::residualPlots - Plots predictors vs residuals and performs curvature test
      • p < 0.05 –> Curvature present and need a quadratic version of the variable
      • Or car::crPlots(model) for just the partial residual plots
    • {ggeffects}
    • Harrell’s {rms}
      • Logistic regression example (link)
        • End of section 10.5; listen to audio

plot(fit)

  • For each fitted model object look at residual plots searching for non-linearity, heteroskedacity, normality, and outliers. Correlation matrix (see bkmk in stats) for correlation and VIF score for collinearity among predictors.
  • If non-linearity is present in a variable run poly function to determine which polynomial produces the least cv error
  • Is heteroscedasticity is present use a square root or log transform on the variable. Not sure if this is valid with multiple regression. If one variable is transformed the others might have to also be transformed in order to maintain interpretability. *try sandwich estimator in regtools, car, or sandwich pkgs*
  • Outliers can be investigated further with domain knowledge or other statistical methods
  • Example: Diamonds dataset from {ggplot2}

    • price ~ carat + cut

      • Bad fit
      • Residuals vs Fitted: Clear structure in the residuals, not white noise. They curve up at the left (so some non-linearity going on), plus they fan out to the right (heteroskedasticity)
      • Scale-Location: The absolute scale of the residuals definitely increases as the expected value increases — a definite indicator of heteroskedasticity
      • Normal Q-Q: Strongly indicates the residuals aren’t normal, but has fat tails (e.g. when they theoretically would be about 3 on the standardised scale, they are about 5 - much higher)

    • log(price) ~ log(carat) + cut

      • Good fit
      • Residuals vs Fitted: Curved shape and the fanning has gone and we’re left with something looking much more like white noise
      • Scale-Location: Looks like solid homoskedasticity
      • Normal Q-Q: A lot more “normal” (i.e. straight line) and apart from a few outliers the values of the standardized residuals are what you’d expect them to be for a normal distribution

Other Diagnostics

  • Misc

  • Check for influential observations - outliers that are in the extreme x-direction

  • Check VIF score for collinearity 

  • For prediction, if coefficients vary significantly across the test folds their robustness is not guaranteed (see coefficient boxplot below), and they should probably be interpreted with caution.

    • Boxplots show the variance of the coefficient across the folds of a repeated 5-fold cv.
    • The “Coefficient importance” in the example is just the coefficient value of the standardized variable in a ridge regression
    • Note outliers beyond the whiskers for Age and Experience
      • In this case, the variance is caused by the fact that experience and age are strongly collinear.
    • Variability in coefficients can also be explained by collinearity between predictors
    • Perform sensitivity analysis by removing one of the collinear predictors and re-running the CV. Check if the variance of the variable that was kept has stabilized (e.g. fewer outliers past the whiskers of a boxplot).

  • step_lincomb - Finds exact linear combinations between two or more variables and recommends which column(s) should be removed to resolve the issue. These linear combinations can create multicollinearity in your model.

    example_data <- tibble(
  a = c(1, 2, 3, 4),
  b = c(6, 5, 4, 3),
  c = c(7, 7, 7, 7)
)

recipe(~ ., data = example_data) |>
  step_lincomb(all_numeric_predictors()) |>
  prep() |>
  bake(new_data = NULL)

# A tibble: 4 × 2
      a     b
  <dbl> <dbl>
1     1     6
2     2     5
3     3     4
4     4     3

ML Prediction

  • Misc

    • Get test predictions from tidymodels workflow fit obj

      preds_tbl <- wflw_fit_obj %>%
    predict(testing(splits)) %>%
    bind_cols(testing(splits), .)

  • GOF

    • RMSE of model vs naive

      preds_tbl %>%
    yardstick::rmse(outcome_var, .pred)
preds_tbl %>%
    mutate(.naive = mean(outcome_var)) %>%
    yardstick::rmse(outcome_var, .naive

  • R2

    preds_tbl %>%
    yardstick::rsq(outcome_var, .pred)
    • Squared correlation between truth and estimate to guarantee a value between 0 and 1

  • Observed vs Predicted plot

    preds_tbl %>%
    ggplot(aes(outcome_var, .pred)) +
    # geom_jitter(alpha = 0.5, size = 5, width = 0.1, height = 0) + # if your outcome var is discrete
    geom_smooth()

  • Feature Importance

    • Example: tidymodel xgboost

      xgb_feature_imp_tbl <- workflow_obj_xgb %>%
    extract_fit_parsnip() %>%
    pluck("fit") %>%
    xgboost::xgb.importance(model = .) %>%
    as_tibble() %>%
    slice(1:20)

xgb_feature_imp_tbl %>%
    ggplot(aes(Gain, fct_rev(as_factor(Feature)))) %>%
    geom_point()