# Measures of work-related illness: Definitions and formulae

The LFS gives estimates of the number of people who have conditions which they think have been caused or made worse by work (regardless of whether they have been seen by doctors). Information is presented as estimated prevalence and rates of self-reported illness and estimated incidence and rates of self-reported illness where:

• Estimated prevalence is the estimated number of people with a work-related illness at any time during the 12-month reference period. It includes the full range of illnesses from long standing to new cases;
• Prevalence rate is defined as the prevalence estimate divided by the population at risk of having a work-related illness;
• Estimated incidence is the estimated number of new cases of work-related illness occurring in the 12 month reference period ie people first becoming aware of their illness in this 12 month period;
• Incidence rate is defined as the incidence estimate (restricted to individuals working in the 12 month period) divided by the population at risk of experiencing a new case of work-related illness during the reference period (generally defined as people working in the last 12 months).

The formulae used to calculate the prevalence estimate and rate of work-related illness relating to individuals ever employed for overall and individual characteristics such as age and sex are given by:

 Annual prevalence estimate of work-related illness for people ever employed = The estimated number of people ever employed with a work-related illness in the 12 month reference period. Note:  This includes illnesses caused or made worse by any job, and includes individuals who have ever worked but are not necessarily employed in the reference period Annual estimated prevalence rate of work-related illness per 100 000 people ever employed = The estimated number of people ever employed with a work-related illness in the 12 month reference period Estimated number of people ever employed x 100 000

Similarly, the formulae used to calculate the prevalence estimate and rate of work-related illness relating to individuals who worked in the last 12 months for overall and individual characteristics such as age and sex are given by:

 Annual prevalence estimate of work-related illness for people who worked in the last 12 months = The estimated number of people who worked in the last 12 months with a work-related illness in the 12 month reference period. Annual estimated prevalence rate of work-related illness per 100 000 people who worked in the last 12 months = The estimated number of people who worked in the last 12 months with a work-related illness in the 12 month reference period Estimated number of people who worked in last 12 months x 100 000

In 2004/05, 2006/07 and 2010/11 surveys, prevalence estimates by occupation and industry relating to individuals ever employed can be calculated. This is since individuals reporting an illness caused or made worse by another job (ie not their current or most recent) were asked to provide occupation and industry details for this other job. It is not possible however to generate prevalence rates for people who have ever worked, as the LFS only collects employment-related information about the current or most recent job in the last 8 years. The formula for prevalence estimate by occupation and industry relating to individuals ever employed is given by:

 Annual prevalence estimate of work-related illness = The estimated number of people ever employed with a work-related illness in the 12 month reference period.

Prevalence rates of work-related illness by employment-related variables such as industry and occupation are necessarily restricted to account for the fact that the LFS only collects employment-related information about the current or most recent job in the last 8 years. However, for 2003/04, 2007/08 and 2008/09 information is only available for illness ascribed to the current or most recent job in the last 12 months. Therefore, for comparison purposes, analysis of employment-related information has been restricted to people working in the last 12 months.

 Annual prevalence estimate of illness caused or made worse by the current or most recent job for people employed in the last 12 months = The estimated number of people with an illness in the 12 month reference period, caused or made worse by the current or most recent job, for people employed in the last 12 months. Note:  The job to which the illness is ascribed may not be the current or most recent, but may still be a job in the last 12 months. This information is not included in the prevalence estimate Cases of illness caused or made worse by the current or most recent job would still be included if the onset of disease was over 12 months ago, while employed in this job, as long as the individual suffered in the reference year (and was still employed in this job at some point during the 12 month reference period) Annual estimated prevalence rate of ill health caused or made worse by the current or most recent job per 100 000 people employed in the last 12 months = The estimated number of people with an illness in the 12 month reference period, caused or made worse by the current or most recent job, for people employed in the last 12 months Estimated number of people employed in the last 12 months x 100 000

The formulae used to calculate work-related illness incidence estimates and rates for overall and individual characteristics such as age and sex are given by:

 Annual incidence estimate of work-related illness for people ever employed = Estimated number of new cases of work-related illness occurring in 12 month reference period, for people who have ever worked Note: This includes illnesses caused or made worse by any job, not just the current or most recent. Annual incidence estimate of work-related illness for people employed in the last 12 months = Estimated number of new cases of work-related illness occurring in 12 month reference period, for people employed in the 12 month reference period Note: This includes illnesses caused or made worse by any job, not just the current or most recent. Annual estimated incidence rate of work-related illness per 100 000 employed in 12 month period = Estimated number of new cases of  work-related illness occurring in 12 month reference period, amongst individuals employed during the 12 month reference period Estimated number of people  employed in the 12 month period x 100 000 Note: As well as including new cases of ill health caused or made worse by the current job the estimate includes new cases caused or made worse by any job as long as the individual was employed in the 12 month period (not necessarily in the job which caused or made the illness worse). Although people will move in and out of work during the 12 month period, people leaving their last (most recent) job in the 12 month reference period will remain at risk of developing a work-related illness during the remainder of the reference period. In addition, people who become employed part way through the period, who have worked prior to the reference period, will be at risk throughout the 12 months. For these reasons, rather than use an estimate of the average working population ie 'worked in the LFS reference week', which is the usual option for dynamic populations, the 'worked in the last 12 months' estimate, which provides a suitable estimate of the people at risk, has been used.

In 2004/05, 2006/07 and 2010/11 surveys, incidence estimates by occupation and industry relating to individuals ever employed can be calculated. This is since individuals reporting an illness caused or made worse by another job (ie not their current or most recent) were asked to provide occupation and industry details for this other job. It is not possible however to generate incidence rates for people who have ever worked, as the LFS only collects employment-related information about the current or most recent job in the last 8 years. The formula for incidence estimate by occupation and industry relating to individuals ever employed is given by:

 Annual incidence estimate of work-related illness for people ever employed = Number of new cases of work-related illness occurring in 12 month reference period, for people who have ever worked.

As for prevalence, incidence rates of work-related illness by employment-related variables such as industry and occupation are necessarily restricted to account for the fact that the LFS only collects employment-related information about the current or most recent job in the last 8 years. However, for 2003/04, 2007/08 and 2008/09 information is only available for illness ascribed to the current or most recent job in the last 12 months. Therefore, for comparison purposes, analysis of employment-related information has been restricted to people working in the last 12 months.

 Annual incidence estimate of ill health caused or made worse by the current or most recent job, for people who worked in the last 12 months = Estimated number of new cases of work-related illness occurring in 12 month reference period, caused or made worse by the current or most recent job, for people who worked in the last 12 months Annual estimated incidence rate of ill health caused or made worse by the current or most recent job per 100 000 people employed in the last 12 months = Estimated number of new cases of work-related illness, occurring in the 12 month reference period, caused or made worse by the current or most recent job, for people who worked in the 12 month reference period Estimated number of people employed in 12 month period x 100 000