Updated as of 09-18-2014 WHO report.
Many reports from on-the-ground workers with the WHO, Doctors Without Borders, state health and aid agencies, etc. have commented that the case and death rates in at least some locations have almost certainly been too low, because of a substantial number of people avoiding going to clinics and hospitals, out of fear primarily. This situation seems to be the worst in Liberia. See this article for example. Today’s WHO-released data from Liberia may be confirmation of this, many new cases and deaths being reported there from August 16-18. Such an explanation could be due to more intensive case tracking/finding. However, it is also possible that the epidemic is simply exploding there now, especially given that it is well established in the capitol of Monrovia. Or it could be due to some combination of the two.
In the graphs below I used a pretty stiff “span” parameter (span = 1.0) in the loess smoothings (dark black lines) of the WHO-reported raw data (thin line). This choice gives about 35 deaths/day in Liberia. If I use something more flexible, span = 0.5 for example, the estimated rates are higher, about 47/day. However, it’s best to go stiff (i.e. conservative) here, because clearly there are major variations due to data gathering and reporting timelines that have been causing large fluctuations in the numbers (discussed more here). But there’s also clearly more than just that going on with this latest surge in numbers.
This situation is now extremely serious, if it wasn’t already. Note also that negative rates early on in the outbreak are presumably due to case retractions or re-classifications. Code generating data and graphs is here and data table itself is here.