Survey population
In June 2007, two areas were particularly badly affected, South Yorkshire and Hull, followed by Worcestershire, Gloucestershire and Thames Valley, in July 2007. The HPA health impact assessments were conducted in South Yorkshire (September - October 2007) and Worcestershire (January - February 2008). These areas were chosen because of the feasibility of carrying out the surveys. The timing of the surveys after the floods varied (3 months after the event in South Yorkshire and 6 months after the event in Worcestershire) due to the logistics of obtaining sampling frames and getting the surveys rolled out.
Local public health staff identified areas that had homes affected in varying degrees by the floods and homes that were not affected. The non-affected households were included as controls. In South Yorkshire, the sampling frame was all addresses in one housing estate (n = 347, population 1,500), all addresses in one village (n = 436, population 1,013), all addresses on the Local Authority flooded properties register in one town (n = 626), and a random selection of 1,252 addresses that were not on the flooded properties register. The town had about 5,000 households, population 12,000.
In Worcestershire, we surveyed all addresses in 2 villages (n = 460), all addresses on the Local Authority flooded properties register in two towns (n = 533) and a random selection of 7,995 addresses (of a total of 12,500) that were not on these registers. As the towns were much larger than the villages in terms of population size (29,000 in the towns compared with 950 in the villages), we sampled 15 properties not on the flooded register for every property that was on the flooded register.
Data collection
A letter of invitation to participate in the survey was sent out to 'the occupier' of households by the Directors of Public Health for the area, outlining a choice of three methods to complete the survey, (i) by telephoning a free phone number and completing the questionnaire on the telephone, or (ii) by completing the questionnaire online or (iii) by return of a paper questionnaire using a freepost envelope that was provided. There was no restriction on who should complete the questionnaire. The questionnaire explained the reason for collecting the data and confirmed that any information provided would be treated with the strictest confidence and provided assurance that participation in the survey was voluntary and would not affect the healthcare received in any way. Due to the low response rate in South Yorkshire to the above three methods of data collection the decision was taken to use face-to-face interviews in the two villages in Worcestershire as these areas were small enough for this method of data collection to be feasible. The invitation letter sent to the two Worcestershire villages explained that data collection would be carried out in face-to-face interviews by interviewers who would visit the houses in the area the following week. Reminder postcards were sent out to non-responders after three weeks.
Exposure variables
We recorded flooding (the presence of water in (i) street or garden outside the house; (ii) basement or cellar of the property or below floor level in the ground floor rooms; (iii) above floor level in ground floor rooms), and damage to property (possessions, furniture, kitchen units, documents or decoration or structures).
Psychological exposure
We measured perceived impact of the floods using a five point likert scale of "much better, better, the same, worse, much worse" in response to the following questions: (i) As a result of the flooding how do you expect your financial circumstances to be? (ii) As a result of the flooding, how do you expect house values will be? We also asked about health concerns relating to the flood: 'Do you have any health worries relating to the flood for yourself or your family?'
Incident management variables
The questionnaire included questions about disruption to essential services (such as gas, electricity and water supply) and evacuation (request to evacuate, a request made and refused, a request made and complied with).
Outcome measures
Psychosocial impact was assessed by psychological distress (General Health Questionnaire-GHQ-12, score of 3 or more), [16] probable generalised anxiety (Generalised Anxiety Disorder GAD-7, score of 10 or more), [17] probable depression (Patient Health Questionnaire PHQ-9, score of 10 or more) [18] and probable post traumatic stress disorder (PTSD) (PTSD checklist-shortform, score of 14 or more) [19]. These are standard epidemiologic instruments with validated criteria indicating increased likelihood of mental health symptoms (screening tools).
Statistical analysis
We examined the proportion of respondents who reported 'damage to property and possessions', 'disruption to services' and each of the variables that measured 'psychological exposure', according to the reported level of water in the home. The five-point Likert scale for 'psychological exposure' variables was collapsed to two categories: (i) "much better", "better" or "the same", and (ii) "worse" or "much worse", as there were very few responses in the "much better", and "better" category or in the "much worse" category.
We first examined the univariate relationship between all the exposure variables and mental health symptoms. Multivariable logistic regression was then used to describe the association between water level in the home and mental health symptoms, adjusted for the following confounding variables: age (≤ 35 yrs, 36-50 yrs, 51 + yrs), sex, presence of an existing medical condition and employment status. We included 'area' and 'method of data collection' variables in the model because of the substantial differences in the socioeconomic status between study areas and variation in data collection methods. 'Psychological exposure' variables and 'incident management' variables (evacuation and disruption to essential services) were then added as explanatory variables to this model. All data analyses were carried out using STATA version 10 [20].