Guinea-Bissau’s armed forces violently seized control of the country’s capital yesterday in an apparent coup d’etat. This is the second successful coup in West Africa in the past month–the other happened in Mali in mid-March–and, if my Twitter feed is any indication, this pair of events has a lot of people wondering if 2012 is going to be an unusually “hot” year for coups in that part of the world.
Statistically speaking, the answer seems to be “no”–or “not yet,” anyway, and it still has a ways to go to get there.
To see if 2012 is shaping up to be a weird year for coup activity in Africa, I used the ‘bcp’ package in R to apply a technique called Bayesian change point detection (BCP) to annual counts of successful and failed coup attempts in the region from 1946 through 2012 (so far). BCP treats time-series data as a collection of independent and identically distributed partitions and looks for points in that series where the data’s generative parameters appear to change. My data on coup events come from the Center for Systemic Peace.
The results are shown below. The top half of the chart plots the observed annual counts (the dots) and the posterior means for those annual counts (the line). The real action, though, is in the bottom half, which plots the posterior probabilities of a change point. The higher that number, the more confident we are that a particular year marks a sudden change. In this series, we see evidence of three change points: one in the mid-1960s, a few years after the start of decolonization; another in the early 1990s, after the end of the Cold War; and a third in the late 1990s, when the rate of coups in the region takes a sharp dip. Meanwhile, the pair of events observed so far in 2012 looks perfectly normal, just about average for the past decade and still well below the recent peak of six events in 2008.
If two coup bids in 2012 does not an aberration make, how many would we need to see this year to call it a significant change? I reran the BCP analysis several times using ever-larger counts for 2012, and it took a big jump to start moving the posterior probability of a change point in any appreciable way. At five events, the posterior probability still hadn’t moved much. At six, it finally moved appreciably, but only to around 0.2. In the end, it took eight events to push the posterior probability over 0.5.
In other words, it would take a lot more than two coup bids in 2012 to mark a significant change from the recent past, and what we’ve seen this year so far looks like normal variation in a stochastic process. Event counts are often noisy, but our pattern-seeking brains still try to find meaning in those small variations. It’s also harder to remember less recent events, and our brains tend to confuse that difficulty with infrequency. It helps to remember those biases whenever a new event starts you thinking about a trend.
NOTE: This version of the plot and the scenario analysis corrects an error in the data used in the original post. For the first run, I forgot that my analysis file ended in 2010, so the 0 events shown for 2011 was a mistake. There were actually two failed coups in Africa last year, one in the DRC in February and another in Guinea in July. With those two events added to the data set, the first third of 2012 looks even more typical than it did before.