Comparison of three methods for ascertainment of contact information relevant to respiratory pathogen transmission in encounter networks

Abstract

Background: Mathematical models of infection that consider targeted interventions are exquisitely dependent on the assumed mixing patterns of the population. We report on a pilot study designed to assess three different methods (one retrospective, two prospective) for obtaining contact data relevant to the determination of these mixing patterns.

Methods: 65 adults were asked to record their social encounters in each location visited during 6 study days using a novel method whereby a change in physical location of the study participant triggered data entry. Using a cross-over design, all participants recorded encounters on 3 days in a paper diary and 3 days using an electronic recording device (PDA). Participants were randomised to first prospective recording method.

Results: Both methods captured more contacts than a pre-study questionnaire, but ascertainment using the paper diary was superior to the PDA (mean difference: 4.52 (95% CI 0.28, 8.77). Paper diaries were found more acceptable to the participants compared with the PDA. Statistical analysis confirms that our results are broadly consistent with those reported from large-scale European based surveys. An association between household size (trend 0.14, 95% CI (0.06, 0.22), P < 0.001) and composition (presence of child 0.37, 95% CI (0.17, 0.56), P < 0.001) and the total number of reported contacts was observed, highlighting the importance of sampling study populations based on household characteristics as well as age. New contacts were still being recorded on the third study day, but compliance had declined, indicating that the optimal number of sample days represents a trade-off between completeness and quality of data for an individual.

Conclusions: The study's location-based reporting design allows greater scope compared to other methods for examining differences in the characteristics of encounters over a range of environments. Improved parameterisation of dynamic transmission models gained from work of this type will aid in the development of more robust decision support tools to assist health policy makers and planners.

Figure 1

Postcode of residence of study participants. Postcode regions in the Melbourne metropolitan area are shaded to indicate the usual residence of study participants. Between 1 and 7 subjects lived in each of the highlighted regions. In addition, two participants came from regional Victoria (one each from Bendigo and Kyneton).

Figure 2

Mixing within and between age groups. The average number of encounters recorded in the paper diary between subjects and contacts in defined age categories is shown for a) all encounters and b) encounters involving physical contact. It should be noted that while averages are reported, the number of observations within each age group upon which these are based varies between 1 (70-79) and 22 (40-49). As can be seen, most contacts occur between individuals of the same age. Further, adults aged between 30-39 years mix with children under 10, presumably in the household. Similarly, 40-49 year olds have a large number of contacts with children and teenagers. Where only encounters involving some physical contact are considered (b), mixing between parents and children becomes more prominent.

Figure 3

Number of individual contacts per day, by source and day of the week. For each subject, the number of recorded contacts per day was determined. For each day of the week studied, we show the median number of contacts recorded in the paper diary, PDA diary and entry questionnaire (Pre Paper). Upper and lower bounds of the boxes reflect the interquartile range, whiskers define upper and lower adjacent values. We also report the mean value above each box plot. We report total contacts (a) and the number of unique names (b).

Figure 4

Non-saturation of contacts over the period of study. The number of uniquely named contacts recorded in the paper diary by each participant (median total of 33) was calculated for each of the three study days. As can be observed, new contacts were still being made by the third study day by the majority of participants.

Figure 5

Location in which encounters took place. The average number of encounters per participant recorded in the paper diary within each age category was described for each of the following locations: 'work', 'travel', 'social', 'shop', 'home' and 'other' on Wednesday (a), Friday (b) and Sunday (c). It should be noted that while averages are reported, the number of observations within each age group upon which these figures are based varies between 1 (70-79) and 22 (40-49). As can be seen from comparison of locations reported on Wednesday (a) and Sunday (c), a predictable shift from predominance of work-based mixing to home-based and social encounters is observed over the weekend. On Wednesdays (a), however, levels of physical contact in the home environment are almost equivalent to those reported in the workplace (data not shown). Wednesday (a) and Friday (b) are more similar.

Figure 6

Correlation between Paper and PDA entries for each participant. Despite variable self report of the difficulty and timeliness of data entry using either paper or PDA, close correlation (0.68) was found between participants' recording of encounters by either means, indicating that both methods are relatively successful in capturing participants encounters (a). The Bland-Altman plot shows a systematic and significant underreporting using the electronic diary compared to paper (b). The horizontal rules show the mean difference (4.52 (0.28, 8.77), and the reference range for difference (-29.7 to 38.8).