Very.

If you have 10 possible independent regressors, and *none of which matter*, you have a good chance to find at least one is important.

A good chance being 40%: prob(one or more looks important) = 1 – prob(non looks important) =

(1)

So, on average, you have around 60% chance to get the correct conclusion. Note that more data does nothing to solve this. Imagine 10 independent trading strategies backtested. Imagine 100. Related post is do-they-really-know-what-they-are-doing where I made the point not to get so excited by a money manager with a very high sharp (or information) ratio.

It is also the reason for my principle point in the R talk I gave in AmsteRdam, know where the profits (losses) stem from, what is the *story* behind what the speculator does.

The following code illustrates the problem, the chance of finding “important” unimportant variables. You can play around with the parameters and double check that increased N does nothing to mitigate the problem.

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options(digits = 3) c0 <- function(TT=100,k = 10, distrib = rnorm){ k <<- k # Write it upwards - we use it later.. y <- distrib(TT) x <- matrix(nrow = TT, ncol = k) for (i in 1:k){ x[,i] = distrib(TT) } lm0 <- list() pv <- NULL signif1 <- NULL for (i in 1:k){ lm0 <- lm(y~x[,i]) pv[i] <- summary(lm0)$coefficients[2,4] # Extract the P.values } return( list(p = sum(pv<.05)/k , signif1 = any(pv<.05) )) } c1 <- c2 <- NULL M <- 500 # Repeat this exercise M times for (i in 1:M){ c1[i] <- c0(TT = 100,k = 10)$p c2[i] <- c0(TT = 100,k = 10)$signif1 } par(mfrow = c(1,1)) plot(c1, ylim = c(0,.9), main = "Probability for finding `important` unimportant", cex.main = 2, ylab = "Probability") abline(a = mean(c1), b = 0, lwd = 3, col = 4) abline(a = sum(c2)/M, b = 0, lwd = 3, col = 2) abline(a = 0.05, b = 0, lwd = 1, col = 4, lty = 2) abline(a = 1-.95^k, b = 0, lwd = 1, col = 2, lty = 2) # here we need k text(x = M/5, y = mean(c1) + 0.02, "Over all false positive..", col = 4, cex = 1.2) text(x = M/4, y = sum(c2)/M + 0.02, "That at least one is false positive..", col = 2, cex = 1.2) legend("topright", c("Simulated prob overall","Simulated prob at least one", "Theoretical prob overall","Theoretical prob at least one"), bty = "n", col = c(2,4,2,4),lty=c(1,1,2,2) ) |

Thanks for the post. Very informative.

There is a good discussion of this issue in Maindonald and Braun Section 6.5. John Maindonald developed the function bestsetNoise() (DAAG package) that can be used to make a similar point.

Tony