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I had a very strange discussion on twitter (yes, another one), about regression curves. I think it started with a tweet based on some xkcd picture (just for fun, because it was New Year's Day)"don't trust linear regressions" https://t.co/exUCvyRd1G pic.twitter.com/O6rBJfkULa— Arthur Charpentier (@freakonometrics) 1 janvier 2017There were comments on that picture, by econometricians, mainly about 'significant' trends when datasets are very noisy. And I mentioned a graph that I saw earlier, a couple of days ago@AndyHarless @mileskimball actually, all that reminds me of a post by @RogerPielkeJr earlier (not a big fan of the regression line) pic.twitter.com/NQgzgVsBcE— Arthur Charpentier (@freakonometrics) 1 janvier 2017Let us reproduce that graph (Roger kindly sent me the dataset)db=data.frame(year=1990:2016,ratio=c(.23,.27,.32,.37,.22,.26,.29,.15,.40,.28,.14,.09,.24,.18,.29,.51,.13,.17,.25,.13,.21,.29,.25,.2,.15,.12,.12))library(ggplot2)The graph is here (with the same aesthetic conventions as Roger's initial graph, i.e. using some sort of barplot)ggplot(db, aes(year, ratio)) +geom_bar(stat="identity") +stat_smooth(method = "lm", se = FALSE)My point was that we miss the 'confidence band' of the regression@freakonometrics @AndyHarless @mileskimball Because it is not a sample. Since 1990 weather losses/global GDP have gone down.— Roger Pielke Jr. (@RogerPielkeJr) 1 janvier 2017In R, at least, it is quite natural to get (and actually, it is the default version of the graph function)ggplot(db, aes(year, ratio)) +geom_bar(stat="identity") +stat_smooth(method = "lm", se = TRUE)It is hard to claim that the 'regression line' is significant (in the sense "significantly non horizontal"). To be more specific, if we look at the output of the regression model, we getsummary(lm(ratio~year,data=db))Coefficients:Estimate    Std. Error t value Pr(&#62;|t|)(Intercept) 9.158531 4.549672  2.013 0.055 .year       -0.004457 0.002271 -1.962 0.061 .---Signif. codes: 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1(which is exactly what Roger used in his graph to plot his red straight line). The p-value of the estimator of the slope, in a linear regression model is here 6%. But I found Roger's point puzzling@freakonometrics @AndyHarless @mileskimball Disagree. U can create one, of course, but doesnt mean much. These data are not balls from urns.— Roger Pielke Jr. (@RogerPielkeJr) 1 janvier 2017See also@freakonometrics @AndyHarless @mileskimball These data are not random.— Roger Pielke Jr. (@RogerPielkeJr) 1 janvier 2017First of all, let us get back to a more standard graph, with a scatterplot, and not bars,ggplot(db, aes(year, ratio)) +stat_smooth(method = "lm") +geom_point() Here, we observe points [latex]\{y_{1990},y_{1991},\cdots,y_{2016}\}[/latex]. In order to draw that blue line, we assume (Econometrics 101, actually) that those observations are realizations of random variables [latex]\{Y_{1990},Y_{1991},\cdots,Y_{2016}\}[/latex]. Randomness here does not come from a survey, or from 'balls in an urn'. Randomness is because hurricanes and floods are themselves seen are realizations of random events. Yes, there might be measurement errors, but that's not where randomness comes from (here). When we talk about 'randomness', it should be related to 'model error' i.e. the error we make if we consider a linear model (here), that is[latex display="true"]?Y_t=\beta_0+\beta_1t+\varepsilon_t[/latex]Even if observations are not obtained from balls in an urn, there is some kind of randomness here. Randomness means that we might have errors (random errors) around the estimated value (that is on the blue curve), [latex]y_t=\widehat{y}_t+\widehat{\varepsilon}_t[/latex]. One might consider a nonlinear model to reduce the error,ggplot(db, aes(year, ratio)) +geom_point() +geom_smooth()but in the case, the danger is to overfitSo yes, when we fit a linear model, there is always some kind of randomness, and it is possible to get a 'confidence band', that will be very useful for predictions (e.g. for reinsurance purpose here).