Table 3. Main Potential Sources of Bias, and Number of Eligible Reports Affected, by Type of Report.

Possible Source Of Bias	Implications	Natural Progression (n = 27)	Duration of Infection (n = 42)	Spontaneous Clearance (n = 4)
Insensitive diagnosis due to microscopy on non-concentrated blood only.	Absence of infection noted during the observation period cannot be taken as proof of clearance.	23	19	4
No certainty about sub-species.	Benign infections could actually be due to transient animal trypanosomes.	25	39	4
Information on symptoms and their duration are based on patient recall.	Patients might systematically over- or under-report the duration of symptoms, or provide inaccurate data.	0	4	0
Information on time of infection based on patient recall.	Patients might not accurately report when they were last exposed to tsetse bites (e.g., patients who had left Africa might not report the most recent trip).	1	29	1
No information about traditional or other treatments during observation period.	Patients might have been cured thanks to traditional therapies or antimicrobials taken for other infections, but which may have limited activity against HAT.	27	22	3
Group of patients is highly self-selected.	Patients with mild infections are more likely to be healthy and thus refuse treatment or be included in natural progression experiments. Patients who remain healthy are more likely to be observed for longer. Measuring the duration of disease based solely on patients who have already died may result in under-estimation. Patients who attend a health centre may be unrepresentative.	6	7	1