## On Statistical Significance and Confidence

#### Posted on 11 August 2010 by Alden Griffith

**Guest post by Alden Griffith from Fool Me Once**

My previous post, “Has Global Warming Stopped?”, was followed by several (well-meaning) comments on the meaning of statistical significance and confidence. Specifically, there was concern about the way that I stated that we have 92% confidence that the HadCRU temperature trend from 1995 to 2009 is positive. The technical statistical interpretation of the 92% confidence interval is this: "if we could resample temperatures independently over and over, we would expect the confidence intervals to contain the true slope 92% of the time." Obviously, this is awkward to understand without a background in statistics, so I used a simpler phrasing. Please note that this does not change the conclusions of my previous post at all. However, in hindsight I see that this attempt at simplification led to some confusion about statistical significance, which I will try to clear up now.

So let’s think about the temperature data from 1995 to 2009 and what the statistical test associated with the linear regression really does (it's best to have already read my previous post). The procedure first fits a line through the data (the “linear model”) such that the deviations of the points from this line are minimized, i.e. the good old line of best fit. This line has two parameters that can be estimated, an intercept and a slope. The slope of the line is really what matters for our purposes here: does temperature vary with time in some manner (in this case the best fit is positive), or is there actually no relationship (i.e. the slope is zero)?

*Figure 1:* Example of the null hypothesis (blue) and the alternative hypothesis (red) for the 1995-2009 temperature trend.

Looking at Figure 1, we have two hypotheses regarding the relationship between temperature and time: 1) there is no relationship and the slope is zero (blue line), or 2) there is a relationship and the slope is not zero (red line). The first is known as the “null hypothesis” and the second is known as the “alternative hypothesis”. Classical statistics starts with the null hypothesis as being true and works from there. Based on the data, should we accept that the null hypothesis is indeed true or should we reject it in favor of the alternative hypothesis?

Thus the statistical test asks: *what is the probability of observing the temperature data that we did, given that the null hypothesis is true*?

In the case of the HadCRU temperatures from 1995 to 2009, the statistical test reveals a probability of 7.6%. Thus there’s a 7.6% probability that we should have observed the temperatures that we did if temperatures are not actually rising. Confusing, I know… This is why I had inverted 7.6% to 92.4% to make it fit more in line with Phil Jones’ use of “95% significance level”.

Essentially, the lower the probability, the more we are compelled to reject the null hypothesis (no temperature trend) in favor of the alternative hypothesis (yes temperature trend). By convention, “statistical significance” is usually set at 5% (I had inverted this to 95% in my post). Anything below is considered significant while anything above is considered nonsignificant. The problem that I was trying to point out is that this is not a magic number, and that it would be foolish to strongly conclude anything when the test yields a relatively low, but “nonsignificant” probability of 7.6%. And more importantly, that looking at the statistical significance of 15 years of temperature data is not the appropriate way to examine whether global warming has stopped (cyclical factors like El Niño are likely to dominate over this short time period).

Ok, so where do we go from here, and how do we take the “7.6% probability of observing the temperatures that we did if temperatures are not actually rising” and convert it into something that can be more readily understood? You might first think that perhaps we have the whole thing backwards and that really we should be asking: “what is the probability that the *hypothesis is true* given the data that we observed?” and not the other way around. Enter the Bayesians!

Bayesian statistics is a fundamentally different approach that certainly has one thing going for it: it’s not completely backwards from the way most people think! (There are many other touted benefits that Bayesians will gladly put forth as well.) When using Bayesian statistics to examine the slope of the 1995-2009 temperature trend line, we can actually get a more-or-less straightforward probability that the slope is positive. That probability? 92%^{1}. So after all this, I believe that one can conclude (based on this analysis) that there is a 92% probability that the temperature trend for the last 15 years is positive.

While this whole discussion comes from one specific issue involving one specific dataset, I believe that it really stems from the larger issue of how to effectively communicate science to the public. Can we get around our jargon? Should we embrace it? Should we avoid it when it doesn’t matter? All thoughts are welcome…

^{1}To be specific, 92% is the largest credible interval that does not contain zero. For those of you with a statistical background, we’re conservatively assuming a non-informative prior.

Stephan Lewandowskyat 09:31 AM on 11 August, 2010Daniel Baileyat 09:32 AM on 11 August, 2010apeescapeat 11:33 AM on 11 August, 2010John Brookesat 19:43 PM on 11 August, 2010John Russellat 22:25 PM on 11 August, 2010realclimate science is put across, provides massive opportunities for the obfuscation that we so often complain about. Please don't take this personally, Alden; I'm sure you're doing your best to simplify -- it's just that even your simplest is not simple enough for those without the necessary background.chris1204at 22:38 PM on 11 August, 2010Bernat 22:41 PM on 11 August, 2010Alden Griffithat 23:05 PM on 11 August, 2010Dikran Marsupialat 23:20 PM on 11 August, 2010andrewcoddat 23:29 PM on 11 August, 2010Dikran Marsupialat 23:36 PM on 11 August, 2010Ken Lambertat 00:00 AM on 12 August, 2010CBDunkersonat 00:09 AM on 12 August, 2010Arkadiusz Semczyszakat 00:12 AM on 12 August, 2010Alden Griffithat 00:59 AM on 12 August, 2010barry1487at 01:01 AM on 12 August, 20101)Generally speaking, the greater the variance in the data, the more data you need (in a time series) to achieve statistical significance on any trend.2)With too-short samples, the resulting trend may be more an expression of the variability than any underlying trend.3)The number of years required to achieve statistical significance in temperature data will vary slightly depending on how 'noisy' the data is in different periods.4)If I wanted to assess the climate trend of the last ten years, a good way of doing it would be to calculate the trend from 1980 - 1999, and then the trend from 1980 - 2009 and compare the results. In this analysis, I am using a minimum of 20 years of data for the first trend (statistically significant), and then 30 years of data for the second, which includes the data from the first. (With Hadley data, the 30-year trend is slightly higher than the 20-year trend) Aside from asking these questions for my own satisfaction, I'm hoping they might give some insight into how a complete novice interprets statistics from blogs, and provide some calibration for future posts by people who know what they're talking about. :-) If it's not too bothersome, I'd be grateful if anyone can point me to the thing to look for in the Excel regression analysis that tells you what the statistical significance is - and how to interpret it if it's not described in the post above. I've included a snapshot of what I see - no amount of googling helps me know which box(es) to look at and how to interpret.Berényi Péterat 01:27 AM on 12 August, 2010CBDunkerson at 00:09 AM on 12 August, 2010We must see rising temperatures SOMEWHERE within the climate system. In the oceans for instance.Nah. It's coming out, not going in recently.Alden Griffithat 01:29 AM on 12 August, 2010Alexandreat 01:29 AM on 12 August, 2010CBWat 01:40 AM on 12 August, 2010CBWat 01:58 AM on 12 August, 2010John Russellat 03:08 AM on 12 August, 2010Chris Gat 05:13 AM on 12 August, 2010Chris Gat 05:25 AM on 12 August, 2010Moderator Response:Rather than delve once more into specific topics handled elsewhere on Skeptical Science and which may be found using the "Search" tool at upper left, please be considerate of Alden's effort by trying to stay on the topic of statistics. Examples of statistical treatments employing climate change data are perfectly fine, divorcing discussion from the thread topic is not considerate. Thanks!apeescapeat 15:08 PM on 12 August, 2010Chris Gat 15:45 PM on 12 August, 2010John Brookesat 16:53 PM on 12 August, 2010Dikran Marsupialat 17:58 PM on 12 August, 2010kdkdat 18:00 PM on 12 August, 2010"Ken is applying a linear test for a positive slope over the most recent 10-12 year period, and, yes, it is failing."It's only failing if you take that data out of context and pretend that the most recent 10-12 period is independent of the most recent 13-50 year period. If you look at the trend of the last decade in context, it's no different to what we observe over the last 50-odd years. I've asked Ken elsewhere quite a few times what's so special about the last decade or so to make him reach his conclusion, but he can't or won't answer the question.Dikran Marsupialat 18:53 PM on 12 August, 2010The Skeptical Chymistat 23:23 PM on 12 August, 2010Ken Lambertat 00:07 AM on 13 August, 2010muoncounterat 01:57 AM on 13 August, 2010anycurve fit, other than as a physical descriptor of what has already taken place? Look back at this graph from On Statistical Significance. It is certainly reasonable to say 'the straight line is a 30 year trend of 0.15 dec/decade'. But this straight line is about as good a predictor as a stopped clock, which is correct twice a day. Superimposed on that trend are more rapid cooling and warming events, which are clearly biased towards warming.muoncounterat 02:02 AM on 13 August, 2010tobyjoyceat 02:20 AM on 13 August, 2010CBWat 02:23 AM on 13 August, 2010tobyjoyceat 04:49 AM on 13 August, 2010kdkdat 06:22 AM on 13 August, 2010Doug Bostromat 07:27 AM on 13 August, 2010Could I dare suggest that it looks cyclical - if not a bit sinusoidal??It has pleasingly smooth curves superimposed on it because of the mathematical treatment and visualization, combined with varying slope. Suggesting it's sinusoidal is indeed daring, some might say even reckless.Berényi Péterat 08:44 AM on 13 August, 201042572455002 39.20 -96.58 MANHATTAN 42572458000 39.55 -97.65 CONCORDIA BLO 42572458001 39.13 -97.70 MINNEAPOLIS 42572551001 40.10 -96.15 PAWNEE CITY 42572551002 40.37 -96.22 TECUMSEH 42572551003 40.62 -96.95 CRETE 42572551004 40.67 -96.18 SYRACUSE 42572551005 40.90 -97.10 SEWARD 42572551006 40.90 -96.80 LINCOLN 42572551007 41.27 -97.12 DAVID CITY 42572552000 40.95 -98.32 GRAND ISLAND 42572552001 40.10 -98.97 FRANKLIN 42572552002 40.10 -98.52 RED CLOUD 42572552005 40.65 -98.38 HASTINGS 4N 42572552006 40.87 -97.60 YORK 42572552007 41.27 -98.47 SAINT PAUL 42572552008 41.28 -98.97 LOUP CITY 42572553002 41.77 -96.22 TEKAMAH 42572554003 40.87 -96.15 WEEPING WATER 42572556000 41.98 -97.43 NORFOLK KARL 42572556001 41.45 -97.77 GENOA 2W 42572556002 41.67 -97.98 ALBION 42572556003 41.83 -97.45 MADISON 42574440000 40.10 -97.33 FAIRBURY, NE. 42574440001 40.17 -97.58 HEBRON 42574440002 40.30 -96.75 BEATRICE 1N 42574440003 40.53 -97.60 GENEVA 42574440004 40.63 -97.58 FAIRMONTIt looks like this on the map (click on it for larger version):Doug Bostromat 09:18 AM on 13 August, 2010kdkdat 09:54 AM on 13 August, 2010kdkdat 09:56 AM on 13 August, 2010HumanityRulesat 11:56 AM on 13 August, 2010barry1487at 13:56 PM on 13 August, 2010rateof warming has increased with each decade over the last 30 years? (I'm trying to think of simple and effective ways to respond to the memes about global temperatures for last 10 - 12 years)Jeff Freymuellerat 14:07 PM on 13 August, 2010Jeff Freymuellerat 14:12 PM on 13 August, 2010kdkdat 14:44 PM on 13 August, 2010Eric (skeptic)at 21:23 PM on 13 August, 2010Berényi Péterat 23:11 PM on 13 August, 2010kdkd at 09:54 AM on 13 August, 2010For reasonable sample sizes parametric statistics are usually good enough.Yes, but you have to get rid of the assumption of normality. Temperature anomaly distribution does get more regular with increasing sample size, but it never converges to a Gaussian. The example below is the GHCN stations from the contiguous United States (lower 48) from 1949 to 1979, those with at least 15 years of data for each month of the year (1718 locations). To compensate for the unequal spatial distribution of stations, I have taken average monthly anomaly for each 1×1° box and month (270816 data points in 728 non-empty grid boxes). Mean is essentially zero (0.00066°C), standard deviation is 1.88°C. I have put the probability density function of a normal distribution there with the same mean and standard deviation for comparison (red line). We can see temperature anomalies have a distribution with a narrow peak and fat tail (compared to a Gaussian). This property has to be taken into account. It means it's way harder to reject the null hypothesis ("no trend") for a restricted sample from the realizations of a variable with such a distribution than for a normally distributed one. Bayesian approach does not change this fact. We can speculate why weather behaves this way. There is apparently something that prevents the central limit theorem to kick in. In this respect it resembles to the financial markets, linguistic statistics or occurrences of errors in complex systems (like computer networks, power plants or jet planes) potentially leading to disaster. That is, weather is not the cumulative result of many independent influences, there must be self organizing processes at work in the background, perhaps. The upshot of this is that extreme weather events are much more frequent than one would think based on a naive random model, even under perfect equilibrium conditions. This variability makes true regime shifts hard to identify.