# Is the variation in the residual standard deviation (on sample) accounted for when one builds a prediction interval (PI)?

This question is somehow related to Is the residual, e, an estimator of the error, $\epsilon$?

I also found some information here: Confidence interval of RMSE

Let's say, I got a model that explains sampled X by mean(X).

Using suggestions from Mr. @whuber I reproduced calculations of the PI from predict.lm in R.

## lm PI

x_dat <- data.frame(x = rnorm(100, 0, 1))

lm_model <- lm(data = x_dat, x ~ 1)

summary(lm_model)

lm_preds <- predict.lm(lm_model
, x_dat
, interval = "prediction"
, level = 0.95
)

beta.hat <- lm_model$coefficients # var-covariance matrix V <- vcov(lm_model) # mean prediction Xp <- model.matrix(~ 1, x_dat) pred <- as.numeric(Xp %*% beta.hat)[1] # mean prediction variance: pred_var <- unname(rowSums((Xp %*% V) * Xp))[1] # confidence t_stat <- qt((1 - 0.95)/2, df = lm_model$df.residual)

# residual MSE

res_mse <- sum(lm_model$residuals ^ 2) / lm_model$df.residual

# PI

PI <- pred + c(t_stat, -t_stat) * sqrt(pred_var + res_mse)

print(PI)

print(lm_preds[1, ])

> print(PI)
[1] -2.043175  2.572508
>
> print(lm_preds[1, ])
fit        lwr        upr
0.2646666 -2.0431747  2.5725079

I only have 2 questions.

Is it right that we assume that the true model error variance is that of sampled residual variance in order to make an unbiased estimate?

Given that we don't know what the true error variance is, does it mean we make a biased estimate of PI, in particular, by not adjusting for an additional dispersion of the residual variance? If so, can we assume that variance of the residual variance follows chi-square distribution in order to get an upper quantile of the value that can be supplied inside for an exact calculation?

predict.lm(...,
pred.var =
)
• The confidence interval for $y(x)$ is a function of the sample. As such, it "includes variation in" every conceivable statistic computed from the sample. In light of this could you clarify your question? Your comment sounds so different from the question as posted that it would behoove you to modify the question substantially. – whuber Mar 15 '18 at 14:08
• Why not just consult the formula? It shows you explicitly what is involved. – whuber Mar 15 '18 at 14:39
• Might I suggest that the R help would be one of the last places you want to rely on for information about statistical procedures? Consider searching our site instead: stats.stackexchange.com/search?q=regression+prediction+interval. – whuber Mar 15 '18 at 16:04
• Here's a search that will turn up the relevant formulas in the top hits: stats.stackexchange.com/…. – whuber Mar 15 '18 at 16:15
• Only if you have a bad prediction interval procedure! – whuber Mar 15 '18 at 18:01

Referencing the initial post, for Q1:  yes, the error variance is estimated (without bias if the model is correct) by an appropriate adjustment to the sample residuals. The bias is corrected with division by a value smaller than $df=n-1$, based on the parameters in the model. (E.g., this is a question students ask a lot: ¿why do we have to divide by $n-2$ instead of $n-1$ for the variance/standard deviation of the residuals for a bivariate model?)  Thus, we have a “reasonable” estimate for the variance

for Q2:  First, if there were bias, then the entire interval would be shifted by the bias...so adjustment via chi-square bounds probably is not appropriate.  Second, to account for the amount of uncertainty, the prediction interval (PI) is slightly elongated.  Again, this references back to the smaller df used to obtain the critical value for this margin of error.

I hope this answers the query...happy to provide more detail if it seems appropriate.

The standard error for a predicted value from a regression (using bivariate example here) is $$\hat\sigma \sqrt{\frac{1}{n} + \frac{(x_0-\bar{x})^2}{(n-1)s_x^2}}$$ where $\hat\sigma^2$ is the estimated residual variance, $n$ is the sample size, $x_0$ is the value at which you want the prediction, and $s_x^2$ is the variance of the $x$ values.  But to clarify, this is the standard error for the predicted value $\hat{y}$ at $x_0$.  As mentioned in Kleinbaum et al. (2014), this can be used to calculate the confidence interval for the parameter (which in this case is the average predicted value at $x_0$).  To account for the fact that we often have to use an estimate for $\sigma$ ($\hat\sigma$ in this case), to obtain the margin of error, we multiple this by a slightly larger multiplication factor to accommodate that extra element of uncertainty.  (In this case, we use a smaller $df=n-2$ which in turn makes $t_{c.v.}$ slightly larger.)

When we move to the next level of estimation, and ask for an interval in which we might observe values of $y$ at $x_0$, then we need to account for the variance of the parameter estimate and the variance of the distribution of errors.  However, by doing so, we are no longer talking about a confidence interval, as we are talking about a random variable and not a parameter; we call this a prediction interval.  Essentially, the variance used in calculating the “margin of error” would be the sum of the variance associated with the parameter estimate and the variance associated with the residuals.  This is captured by adding a one under the radical: $$\hat\sigma \sqrt{1 + \frac{1}{n} + \frac{(x_0-\bar{x})^2}{(n-1)s_x^2}}$$ Conceptually, the 1 “adds” the estimate for $\sigma$ to the standard error.

• Could you, please, elaborate more on the connection between the variance in linear combination of inputs multiplied by model weights, and the broadening of the PI? On population we don't have this source of variance. We only have the model error variance. While on a sample we have this source of variance (sampling variance) atop of the residual variance. So it comes out in the end that the CI of the prediction (not counting residuals) needs to be taken in the account when talk about the best estimate for the PI (which is related to model errors only). – Alexey Burnakov Mar 21 '18 at 15:37
• I'll attempt to address this in Addendum #1 to my answer above. – Gregg H Mar 21 '18 at 21:21
• Full reference for Addendum #1: Kleinbaum, Kupper, Nizam & Rosenberg (2014). Applied regression analysis and other multivariable methods, 5th ed. Cengage. A more detailed explanation can be found on pp.69ff. – Gregg H Mar 21 '18 at 21:42
• thank you a lot. In the last equation, should not the σ be σ-hat? And also I wonder if this equation is for the intercept-only case, as there is no other coefficients involved? – Alexey Burnakov Mar 22 '18 at 10:02
• Good catch...that would be an estimated value (with hat). This example is for a bivariate (single variable, with slope & intercept). Happy to provide the matrix form for multiple regression models. – Gregg H Mar 22 '18 at 10:34

I think the truth is that I thought incorrectly.

The following simulation shows that under normal error assumption (normal X actually) the proportion of examples falling out of a 95% PI on a population is not different from 5%.

If I make something stupid, please correct / post an answer.

start_time <- Sys.time()

popul <- 1000000
sam_size <- 25
sim_num <- 10000

big_x <- rnorm(popul, 0, 1)

outofpi <- numeric()

for(i in 1:sim_num)
{

sam_ind <- sample(popul, sam_size, replace = F)

x_dat <- data.frame(x = big_x[sam_ind])

lm_model <- lm(data = x_dat, x ~ 1)

lm_preds <- predict.lm(lm_model
, x_dat
, interval = "prediction"
, level = 0.95
)[1, ]

newd <- big_x[-sam_ind]

outofpi[i] <- length(newd[newd < lm_preds[2] | newd > lm_preds[3]]) / length(newd)

}

Sys.time() - start_time

hist(outofpi, breaks = 'fd')

t.test(x = outofpi
, alternative = "greater"
, mu = 0.05
, conf.level = 0.95
)

One Sample t-test

data:  outofpi
t = 0.54362, df = 9999, p-value = 0.2934
alternative hypothesis: true mean is greater than 0.05
95 percent confidence interval:
0.04961823        Inf
sample estimates:
mean of x
0.05018844