NHS Performance Indicators : July 2000 How to interpret the graphs

NB: The performance indicators are presented in graphical format. Due to changes in the method to calculate the indicator values, the information in this publication should not be compared with those published last year.

All the graphs in this document are presented as bar charts. There are two main presentational styles.

In the first style, as in graph 1i, the organisation name is listed down the left hand side and the indicator values are presented in the form of a bar. The scale is shown along the bottom and the indicator value can be read off from this scale.

The unit of the scale can vary. In some instances this is a percentage or a rate per head of population in other instances this is a number of occurrences.

Some of the performance indicators are composite indicators. This is where two or more indicators are pooled together to give one overall indicator. The unit of the scale will therefore show a composite value for these indicators.

In the second style, as in graph 3vii, the organisation name is listed down the left hand side and the indicator values are presented in the form of a dot with 95% confidence intervals around this value represented as lines. Again, the scale is shown along the bottom and the indicator value can be read off from this scale.

The confidence intervals assess the level of uncertainty caused by chance occurrences in the indicator value. The 95% confidence interval gives the range in which we would expect the true indicator value to fall 95 times out of 100. In interpreting these types of graphs it should be noted that if a Trust's or Health Authority's confidence intervals do not overlap with the England rate, it is likely that their indicator values are genuinely different from the national rate.

Various calculations have been undertaken to facilitate 'like' with 'like' comparisons, these are summarised below.

Local populations vary in their age structures. For instance, retirement towns have more elderly people than other areas. This could result in higher rates of emergency readmission to hospitals in these towns, even if there were no differences in the quality of care. To avoid comparisons being distorted the indicators have been adjusted for differences in the age and sex structure. The method used for age and sex standardisation is available here.

There are differences in the social and economic mix of local populations, which will be reflected in the numbers and types of patients admitted to hospital. Social and economic factors may influence both health and the results of health care, irrespective of the quality of care. However, where there is evidence that a service is effective or that a certain standard should be met, then it should be applied equally to all patients irrespective of their socio-economic status. Social and economic influences may explain but do not necessarily justify variation. Adjustment of the indicators for socio-economic variation may mask the very information on inequalities that the NHS should know about and address. Therefore no attempt has been made to adjust the indicators for social and economic characteristics.

There are different types of hospitals ranging from small community hospitals to large teaching hospitals. Consequently their patients differ in terms of the severity of their conditions (case-mix). Such variations will affect treatment outcomes irrespective of the quality of care. Therefore comparisons between hospitals of the same type are more appropriate than comparisons between different types. In order to facilitate 'like' with 'like' comparisons, NHS hospital Trusts have been grouped into similar types. Lists of all NHS Trusts and type of hospital cluster to which they are allocated are available from the Downloadable files page.

It is likely that there will be less variation in case-mix between resident populations of Health Authorities than between smaller hospital patient populations, therefore Health Authorities are not grouped into clusters.

Many of the indicators are based on Hospital Episode Statistics data supplied to the Department of Health by the NHS. Detailed specifications of the HES data used in the indicators is available from the Downloadable files page. This data set is not always complete and missing information impairs the reliability of comparisons. Therefore data quality analyses were undertaken. See the Data Quality sections (on the main contents page) for detail. On the basis of these analyses, every Health Authority and every NHS hospital Trust was given a data quality mark for each year. The graphs in this publication only include organisations which met certain standard criteria in terms of data completeness and validity of coding. Those with "poor" data quality have been excluded from the graphs. Trusts may also be excluded from the graphs for a number of other reasons. These include:

• Trusts with low levels of activity for the indicator which would give unrepresentative results;
• All 'priority single service' trusts are excluded from all indicators as the indicators are not relevant to the patients they treat.
• Other clusters may be excluded from individual indicators for similar reasons.

A small number of trusts also identified in advance that they had problems with the quality of their data and stated that their information was not suitable for the calculation of clinical indicators. These Trusts were all either priority single service trusts, which are excluded from the indicator calculations in this document, or specialised community trusts for whom data on two indicators only are relevant. In the process of compiling this document, a small number of Trusts found local data recording problems. The results should therefore be treated with some caution. However, generally the quality of data recording and data completeness has improved considerably from the previous year.

More detailed explanations of the graphs and the underlying methodological issues are available under the Technical Specifications section.

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Last updated July 2000.