The study will use the Danish Occupational Hospitalisation Register (OHR), a database obtained through a record-linkage between three national registers--the central person register, the hospital register, and the employment classification module. Currently, the OHR includes every person who has been economically active and an inhabitant of Denmark sometime after 1980.
The central person register contains information on gender, addresses and dates of birth, death and migrations for every person who is or has been an inhabitant of Denmark sometime between 1968 and present time. A person's occupation and social status are, since 1975, registered annually in the employment classification module [
10]. The occupations are, since 1994, coded in accordance with Statistics Denmark's Standard Classification of Occupations (DISCO-88) [
11], which is a national version of the International Standard Classification of Occupations (ISCO-88). Socio-economic status is coded in accordance with Statistics Denmark's official socio-economic classification [
12]. At the one- and two-digit level, the classification contains the following social groups:
1.
Gainfully occupied people
Inclusion criteria
A person will be included in the study if he/she
1. on at least one occasion in the time period 1996 - 2006, was registered in the hospital patient registry with one of the following ICD-10 codes as principal diagnosis:
• I60 subarachnoid haemorrhage
• I61 intracerebral haemorrhage
• I63 cerebral infarction
• I64 stroke, not specified as haemorrhage or infarction
2. belonged to the age interval 20 - 57 years at the time of the hospital contact
3. was gainfully occupied the year preceding the hospital contact
Statistical analysis
The study will consist of two parts, one is descriptive while the other utilises statistical inference techniques to test hypotheses and estimate odds ratios for RTW (return to work).
In the descriptive part we follow the stroke patients for five calendar years after the stroke and register their main social status in each of these years. This part of the study will only include people who were less than 55 years at the time of the stroke. For any given patient, the calendar year of the stroke will be defined as year 0, the next calendar year will be defined as year 1 etc.
In the regression analysis, we will look at the odds of having a socio-economic code, which indicates gainful occupation in year 2 after stroke. The outcome variable is set to 1 if the person is self-employed, assisting spouse or employee in that particular year. It is set to 0 if the person is unemployed, not economically active or dead.
As explanatory variables we will use gender, age, diagnosis, calendar year, occupational class, self-employment, and type of municipality.
Age at the time of the stroke will be divided into the categories 20 - 49 years and 50 - 57 years. In Denmark it is possible to retire at the age of 60, regardless of health condition. This is why we do not include people who would be older than 60 years at follow-up. The cut-point 50 years conforms to OECD's definition of older workers [
13], who are known to have a more insecure labour market attachment than the younger ones.
The variable 'Diagnosis' contains the four stroke categories given in the section 'inclusion criteria'.
The variable 'Self-employment' is set to 1 if the person is self-employed or assisting spouse and 0 if he/she is an employee, the year preceding the stroke.
The variable 'Occupational class' is based on the first digit of the DISCO-88 classification the year preceding the stroke. It contains the following categories:
• Legislators, senior officials and managers (DISCO-88 group 1)
• professionals (DISCO-88 group 2)
• technicians and associate professionals (DISCO-88 group 3)
• workers in occupations that require skills at a basic level (DISCO-88 group 4 - 8)
• workers in elementary occupations (DISCO-88 group 9)
• gainfully occupied people with an unknown occupation (missing DISCO-88 code)
The variable 'Type of municipality' is set to 1 if the person lived in a municipality which had a brain injury rehabilitation centre at the time of the stroke. Otherwise it is set to 0. The following municipalities had a brain injury rehabilitation centre throughout the study period: Copenhagen, Odense, Aarhus, Roskilde, Aalborg and Vejle.
There are at least two reasons for believing that the RTW probabilities depended on which calendar year the stroke occurred. Firstly, quality of stroke treatment and rehabilitation has a tendency to improve with time. This is illustrated by two Finish studies which show that 28-day case fatality rates among 35 - 74 year-old stroke patients decreased by approximately three percent annually in the time period 1983 - 2001 [
14,
15]. A similar trend was observed in England [
16]. Secondly, a series of political initiatives and legislative changes, aimed at improving return to work rates in Denmark, occurred during the study period. In 1998, the flexi-job system was introduced, which allows people with permanently reduced work capacity to work part time, yet through public subsidy get full salary. In 2001, the flexi-job system was reformed. A person who is eligible to a flexi-job would be entitled to unemployment benefits if no such work could be found. In 2003, a new disability retirement scheme was introduced. The main intent of the reform was to ascertain that as many as possible retain their attachment to the labour market. A new procedure for assessing work capacity was introduced, and disability pension would only be granted if work ability was permanently reduced and flexi-job work was unfeasible. In 2005, the municipal control of the sick-listed was intensified by a change in the Sickness Benefit Act. Follow-up evaluations were required once a month instead of once every second month, and a reintegration plan was to be drawn up after four instead of six months [
17]. To deal with this possible time dependency, we incorporate calendar year into the model as a class variable.
It is also reasonable to believe that RTW probabilities depend on place of residence. In Denmark, municipalities play an important role in the return to work process. According to the law, it is the municipal officer and not the physician who has the formal right to decide whether or not a person qualifies for sickness benefit, disability pension, or vocational rehabilitation. The law also stipulates that the municipality should perform regular follow-up evaluations and draw up detailed reintegration plans for each sick-listed citizen at risk of long-tem sickness absence [
17]. RTW initiatives are often launched at the municipality level and some municipalities might be more active than others. The unemployment situations may also differ between municipalities. In the present study, we will use a multi-level analysis to deal with intra-municipality correlations -- the municipalities are treated as the subjects while the individual observations within the municipalities are treated as correlated repeated measurements.
The logistic regression will be performed by use of the GENMOD procedure in SAS version 9.1. Only main effects are considered. An exchangeable correlation structure is assumed. The empiric standard error estimates will be used.
Table
1 shows how the results from the first part of the study will be presented. Table
2 will list odds ratios and P-values that will be estimated in the second part of the study. Odds ratio for return to gainful occupation two year after stroke, by onset calendar year, will be illustrated by a graph. The calendar year 1996 will be used as reference. All hypotheses are two-tailed. A P-value will be deemed statistically significant if it is less than 0.05.
Table 1
Social group distribution (%) by time passed since onset of illness, among stroke patients in Denmark who were 20-54 year of age and gainfully occupied at the time of the stroke
Self-employed people | | | | | |
Assisting spouses | | | | | |
Employees | | | | | |
People on unemployment benefits | | | | | |
People in training/education | | | | | |
Pensioners | | | | | |
Other not economically active | | | | | |
Deceased | | | | | |
Total | 100 | 100 | 100 | 100 | 100 |
Table 2
Odds ratios (OR), with 95% confidence interval (CI) for return to gainful occupation two year after stroke
Gender (P = xxx) | Men | | 1.00 | - |
| Women | | | |
Age (P = xxx) | <50 years | | 1.00 | - |
| 50 - 57 years | | | |
Diagnosis (P = xxx) | Subarachnoid haemorrhage | | | |
| Intracerebral haemorrhage | | | |
| Cerebral infarction | | 1.00 | - |
| Stroke, not specified as haemorrhage or infarction | | | |
Self-employment (P = xxx) | No | | 1.00 | - |
| Yes | | | |
Occupational class (P = xxx) | Legislators, senior officials and managers | | | |
| Professionals | | | |
| Technicians and associate professionals | | | |
| Workers in occupations that require skills at a basic level | | | |
| Workers in elementary occupations | | 1.00 | - |
| Gainfully occupied people NOS | | | |
Municipality type (P = xxx) | Municipality without brain injury centre | | 1.00 | - |
| Municipality with brain injury centre | | | |