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A meta-analysis, also known as a systematic review, is a statistical procedure in which the
PRINCIPLES AND PRACTICE OF CLINICAL PARASITOLOGY
results of previous research are integrated, with the aim of being able to resolve issues that cannot be concluded from a single study alone. It addresses: (a) the formal synthesis of the results of independent studies to yield a quantitative estimate of the overall size of the response parameter, supported by the larger sample sizes afforded by combining individual studies; and (b) the quantification and investigation of sources of heterogeneity among studies. In medical sciences, the intensifying use of meta-analysis has coincided with the increasing focus of medical research on the randomized clinical trial (Peto, 1987), the subject undoubtedly benefiting from the rising level of concern about the interpretation of small and individually inconclusive clinical trials. The use of meta-analysis, however, is not confined to the synthesis of information from experimental studies alone, and a number of studies that involve the meta-analysis of nonexperimental data have been published in recent years (Petitti, 1994). More recently, the method has also been applied to the synthesis of ecological and evolutionary data, including data on parasitic infections (Arnqvist and Wooster, 1995; Poulin, 1996; Michael et al., 1994). Here, we will address the use of meta-analysis for gaining a better understanding of the population biology of parasitic infections based on examples from Poulin (1996) and Michael et al. (1994). Readers interested in the more traditional use of meta-analysis in summarizing and integrating results from randomized clinical trials in tropical medicine are referred to the excellent systematic reviews addressing the efficacy of different treatments for various parasitic diseases, made available electronically by the Cochrane Parasitic Diseases Group at the Cochrane Collaboration website (Germany http://www.imbi.uni-freiburg.de, UK: http:/www.cochrane.co.uk).
The Effect of Gender on Helminth Infections
Poulin (1996) employed a fixed effects metaanalysis (Hedges and Olkin, 1985) to investigate whether there was a consistent host sex bias in infection levels (in terms of both prevalence and intensity) with helminth parasites. The analysis was carried out by comparing published data on
parasite burdens between female and male vertebrates. Evidence of a bias in favour of one sex would suggest that higher levels of parasitism may be a relative cost associated with that sex and could have a range of evolutionary implications.
Data for the meta-analysis were obtained from a total of 85 published studies and yielded a total of 295 comparisons of prevalence and 169 comparisons of intensity. Some species were involved in more than one comparison, as certain host species harboured more than one parasite species and certain parasite species infected more than one host species. Host species involved in the comparisons represented several families, although no host taxon was involved in a disproportionate number of comparisons. However, among parasites, digeneans and especially nematodes were well represented in the data set. Comparisons of prevalence and intensity of infection between the sexes were computed for each set of values, essentially to obtain standardized effect size measures that are independent of sample size (Hedges and Olkin, 1985). For prevalence, differences were calculated using the following formula:
(Pf - Pm)(J) (2)
J = 1 - 4(Nf + Nm - 2)-1
which is simply the difference between the prevalence in females (Pf) and that in males (Pm) weighted by J, which is a correction for small sample sizes or numbers of individuals examined (Nf and Nm). As total sample size increases, J will approach 1, such that more weight is given to comparisons based on larger sample sizes. The comparison was computed to give positive values when prevalence is greater in females but negative values when greater in males. Similarly, differences in intensities were computed as:
(/f - Im)J (3)
which is again the difference between the mean intensity in females (If) and that in males (Im) corrected for sample size (here denoting the
Fig 2.6 Frequency distribution of differences in parasite prevalence from host-parasite systems involving fish, bird and mammal hosts. Arrows indicate the arithmetic mean difference. Values to the left of the broken line denote higher prevalence in males, while values to the right indicate higher prevalence in female hosts. From Poulin (1996), with permission of the University of Chicago Press
numbers of infected individuals). Differences in intensity were expressed as a proportion of the intensity in females to standardize for the variability in the mean intensities recorded, which ranged from a few parasites to several thousand parasites per host. If there is no sex bias
in levels of infection, differences in prevalence and infection are expected to be normally distributed around a mean of zero. Also, the number of positive differences (higher levels of infection in females) should equal the number of negative ones (higher infection in males).