Among the selected articles several different outcome measurements were used to study the effect of drinking water. Information about GI was collected via health-care systems, from proxy data or directly from studied cohorts. By proxy data we here refer to indicators which are likely to correlate with GI incidence in the community. Several of the included studies used internet search volumes or drug sales data. In the cohort studies GI was measured as self-reported symptoms. Different definitions of gastrointestinal disease were used in different studies and this may affect comparability between studies. For self-reported GI there is a higher degree of recall bias if longer recall periods are used. Some symptoms, such as nausea, can also be subjective and difficult to measure. Among the included papers we identified two commonly used case definitions: acute gastrointestinal illness (AGI) and highly credible gastrointestinal illness (HCGI) (Table 2). The different methodologies and types of data are summarized in Table 3.
Case definitions of self-reported GI in different studies
| Illness . | Definition . | References . |
|---|---|---|
| AGI | Vomiting and/or diarrhoea with at least three loose stools during a 24-h time-period | Nygård et al. (2007), Febriani et al. (2010), Borchardt et al. (2012) |
| HCGI | Any of the following conditions: (1) vomiting, (2) watery diarrhoea, (3) soft diarrhoea and abdominal cramps, (4) nausea and abdominal cramps | Payment et al. (1991, 1997), Colford et al. (2002, 2005, 2009), Frost et al. (2009) |
| Illness . | Definition . | References . |
|---|---|---|
| AGI | Vomiting and/or diarrhoea with at least three loose stools during a 24-h time-period | Nygård et al. (2007), Febriani et al. (2010), Borchardt et al. (2012) |
| HCGI | Any of the following conditions: (1) vomiting, (2) watery diarrhoea, (3) soft diarrhoea and abdominal cramps, (4) nausea and abdominal cramps | Payment et al. (1991, 1997), Colford et al. (2002, 2005, 2009), Frost et al. (2009) |
Overview of methodologies and data sources for investigating relationship between drinking water and sporadic GI
| Methodology . | Definition . | Data sources . | Advantages . | Limitations . |
|---|---|---|---|---|
| Ecological study | Observational study of the correlation between risk factors and health outcomes based on populations defined geographically and/or temporally | Pharmacies | Available in most countries | Low correlation with sporadic GI |
| International codes allow the use of data for specific drugs and facilitate comparisons between countries | No information about water consumption or confounders | |||
| Affected by opening hours and holidays | ||||
| Telephone triage | Lots of data (high statistical power) | Only available in some countries | ||
| High correlation with endemic GI | ||||
| Description of symptoms | Reporting bias | |||
| Seasonal and geographical variation | ||||
| Health care | Available in most countries | High under-reporting | ||
| Diagnosed cases | Mostly severe cases of GI | |||
| Large study population required | ||||
| Case-control study | Observational study in which two existing groups differing in outcome are identified and compared based on their association with hypothesized risk factors | Health care | Diagnosed cases | High under-reporting |
| Allows identification of risk factors | Mostly severe cases of GI | |||
| Selection bias | ||||
| Prone to recall bias | ||||
| Cross-sectional study | Observational study that analyses data collected from a population, or a representative subset, at a specific point in time | Questionnaires | Possibility to collect detailed data on symptoms, water consumption and confounders | Only point estimate |
| Interviews | Recruitment bias | |||
| Low response rate | ||||
| Self-reporting may be subjective | ||||
| Cohort study | Observational study that analyses data collected from a population, or a representative subset, over a period of time | Questionnaires | Possibility to collect detailed data on symptoms, water consumption and confounders | Expensive |
| Collection of data over time | Labour intensive | |||
| Interviews | Recruitment bias | |||
| Health diaries | Low response rate | |||
| Patient registers | Self-reporting may be subjective | |||
| Household intervention | An experimental study measuring changes in risk following a risk reducing measure in a population | Questionnaires | Possibility to collect detailed data on symptoms, water consumption and confounders | Expensive |
| Interviews | Collection of data over time | Labour intensive | ||
| Health diaries | Direct measurement of risk reduction due to intervention | Excludes non-home owners | ||
| Different risk factors cannot be elucidated with point of use intervention |
| Methodology . | Definition . | Data sources . | Advantages . | Limitations . |
|---|---|---|---|---|
| Ecological study | Observational study of the correlation between risk factors and health outcomes based on populations defined geographically and/or temporally | Pharmacies | Available in most countries | Low correlation with sporadic GI |
| International codes allow the use of data for specific drugs and facilitate comparisons between countries | No information about water consumption or confounders | |||
| Affected by opening hours and holidays | ||||
| Telephone triage | Lots of data (high statistical power) | Only available in some countries | ||
| High correlation with endemic GI | ||||
| Description of symptoms | Reporting bias | |||
| Seasonal and geographical variation | ||||
| Health care | Available in most countries | High under-reporting | ||
| Diagnosed cases | Mostly severe cases of GI | |||
| Large study population required | ||||
| Case-control study | Observational study in which two existing groups differing in outcome are identified and compared based on their association with hypothesized risk factors | Health care | Diagnosed cases | High under-reporting |
| Allows identification of risk factors | Mostly severe cases of GI | |||
| Selection bias | ||||
| Prone to recall bias | ||||
| Cross-sectional study | Observational study that analyses data collected from a population, or a representative subset, at a specific point in time | Questionnaires | Possibility to collect detailed data on symptoms, water consumption and confounders | Only point estimate |
| Interviews | Recruitment bias | |||
| Low response rate | ||||
| Self-reporting may be subjective | ||||
| Cohort study | Observational study that analyses data collected from a population, or a representative subset, over a period of time | Questionnaires | Possibility to collect detailed data on symptoms, water consumption and confounders | Expensive |
| Collection of data over time | Labour intensive | |||
| Interviews | Recruitment bias | |||
| Health diaries | Low response rate | |||
| Patient registers | Self-reporting may be subjective | |||
| Household intervention | An experimental study measuring changes in risk following a risk reducing measure in a population | Questionnaires | Possibility to collect detailed data on symptoms, water consumption and confounders | Expensive |
| Interviews | Collection of data over time | Labour intensive | ||
| Health diaries | Direct measurement of risk reduction due to intervention | Excludes non-home owners | ||
| Different risk factors cannot be elucidated with point of use intervention |