Methodology Document, 2017-2018 
Published 13th June 2019 

## Contents

| Introduction                                | 3   |
|---------------------------------------------|-----|
|                                             |     |
| How the data is collected                   | 4   |
|                                             |     |
| What is done with the data after submission | 5   |
|                                             |     |
| Data cleaning                               | 5   |
|                                             |     |
| Validation                                  | 6   |
|                                             |     |
| Production of Analysis database             | 6   |
|                                             |     |
| Analysis                                    | 8   |
|                                             |     |
| Coverage                                    | 8   |
|                                             |     |
| Data period                                 | 9   |
|                                             |     |
| Registrations                               | 9   |
|                                             |     |
| Definition of the Care Processes            | 9   |
|                                             |     |
| Definition of the Treatment Targets         | 11  |
|                                             |     |
| Standardisation                             | 13  |
|                                             |     |
| Structured Education                        | 14  |
|                                             |     |
| Learning Disabilities                       | 14  |
|                                             |     |
| Disclosure Control                          | 15  |
|                                             |     |
|                                             |     |
|                                             |     |

## Introduction

The National Diabetes Audit (NDA) is managed by the Healthcare Quality Improvement Partnership (HQIP) on behalf of NHS England and delivered by NHS Digital formerly the Health and Social Care Information Centre (HSCIC), working in collaboration with Diabetes UK and Public Health England (PHE). The NDA is a major national clinical audit, which measures the effectiveness of diabetes healthcare against NICE Clinical Guidelines and NICE Quality Standards, in England and Wales. The NDA collects and analyses data for use by a range of stakeholders to drive changes and improvements in the quality of services and health outcomes for people with diabetes. The NDA answers five key questions: 

1. 
Is everyone with diabetes diagnosed and recorded on a practice diabetes register?  
2. 
What percentage of people registered with diabetes received the NICE key processes of diabetes care?  
3. 
What percentage of people registered with diabetes achieved NICE defined treatment targets for glucose control, blood pressure and blood cholesterol?  
4. 
What percentage of people registered with diabetes are offered and attend a structured education course? 
5. 
For people with registered diabetes what are the rates of acute and long term complications (disease outcomes)? 
 The NDA supports improvement in the quality of diabetes care by enabling participating NHS services and organisations to: 

- assess local practice against NICE guidelines - compare their care and care outcomes with similar services and organisations - identify gaps or shortfalls that are priorities for improvement - identify and share best practice - provide comprehensive national pictures of diabetes care and outcomes in 
England and Wales 
Through participation in the audit, local services are able to benchmark their performance and identify where they are performing well and improve the quality of treatment and care they provide. On a national level, wide participation in the audit also provides an overview of the quality of care being provided in England and Wales. A Quality Improvement Toolkit has been developed in collaboration with the RCGP to help practices use their diabetes data to improve services http://www.rcgp.org.uk/clinical-andresearch/toolkits/quality-improvement-toolkit-for-diabetes-care.aspx(Opens in a new window)  

This document provides you with the information and links you need to understand how the audit data is collected and analysed. 


## How The Data Is Collected

Information is collected from GP practice administrative data via pre-agreed extracts of their computer system. Data is extracted from General Practice clinical systems and specialist diabetes service units in secondary care hospitals. For the 2017-18 audit collection, a primary care specification was developed containing all the read codes for extraction and information about time periods the data needed to capture, which was used as the basis for each system supplier extract. For the first time, the 2017-18 NDA primary care collection was delivered via the General Practice Extraction System (GPES). GP practices were invited to participate in the NDA through their GP clinical system via the Calculating Quality Reporting Service (CQRS). By accepting the invitation, practices opted into GP system supplier-developed queries to automatically extract their NDA data, based on the NDA primary care specification. The NDA operates under an 'opt in' model to remain open and transparent with practices and services about what data are being collected. System suppliers went through a testing and certification process with GPES prior to the final data extraction to highlight any issues with the data and ensure the data that was extracted was acceptable and aligned to the primary care specification. Data were extracted and supplied to the Clinical Audit and Registries Management Service in XML format. For Wales, GP practices confirmed that they wanted to participate in the NDA and NHS Wales Informatics Service collected the data on their behalf and transferred it to NHS Digital. For specialist services, an Excel template specification was available for them to complete. Guidance was given for which data items needed to be returned and the corresponding date ranges. Services worked with their IT departments and Audit Teams to develop bespoke extracts from their administrative systems to be able to return the data. Where Services were not able to electronically extract the data, it was still possible for them to participate in the NDA by submitting a list of the NHS numbers of the patients registered to their clinics which is routinely available from their Patient Electronic System (PAS). This was then matched to primary care records to generate a report for the specialist services. The audit is collected and disseminated in England under S254 of the Health and Social Care Act 2012, known as a Direction. This means in England it is mandatory for GP practices and specialist services to participate. In Wales the Confidentiality Advisory Group has granted permission to collect the NDA under Regulation 5 of the Health Service (Control of Patient Information) Regulations 2002 (known as Section 251 support), it is voluntary for GP practices and specialist services in Wales to participate in the NDA. The NDA participation stood at 98.3 per cent of all GP practices in England and Wales for 2017-18. Further information on submitting data to the NDA can be found here. The information collected from GP practices for the audit are individual level data and so contain demographic information such as age, sex, ethnicity, diabetes duration and some geographic variables such as IMD centiles and postcode.   

## Technical Specifications Of The Collection Can Be Found Here.

For NDA 2017-18, for the first time, information on drugs prescribed by GPs has been collected. The data covers antihypertensive drugs, statins and diabetes medications. 

## What Is Done With The Data After Submission

Data cleaning  
Cleaning rules are applied to supplied data to ensure valid information is included in the audit. The following cleaning rules are applied. 

Data outside acceptable limits is deemed to be invalid and set to unknown or null. 

| Rule statement       | Valid codes and values                                               | Invalid                                            |
|----------------------|----------------------------------------------------------------------|----------------------------------------------------|
| codes and            |                                                                      |                                                    |
| values set           |                                                                      |                                                    |
| to unknown           |                                                                      |                                                    |
| values               |                                                                      |                                                    |
| SEX                  | 1,2,9                                                                | 0                                                  |
| ETHNICITY            | A, B, C, D, E, F, G, H, J, K, L, M, N, P, R, S, T, W, Z              | NULL                                               |
| DIABETES_TYPE        | 01, 02, 06, 08                                                       | 99                                                 |
| ALBUMIN_TEST         | 01, 02, 03, 04, 99                                                   | NULL                                               |
| ALBUMIN_STAGE        | 01, 02, 03, 99                                                       | NULL                                               |
| EYE_EXAM_VALUE       | 01                                                                   | 99                                                 |
| FOOT_EXAM_VALUE      | 01                                                                   | 99                                                 |
| SMOKING_VALUE        | 1, 2, 3, 4, 9                                                        | NULL                                               |
| ED_REVIEW_VALUE      | 01                                                                   | 99                                                 |
|                      |                                                                      |                                                    |
| ED_REVIEW_DATE       | NULL                                                                 |                                                    |
| ED_OFFER_VALUE       | 01                                                                   | 99                                                 |
|                      |                                                                      |                                                    |
| ED_OFFER_DATE        | NULL                                                                 |                                                    |
| ED_ATTEND_VALUE      | 01                                                                   | 99                                                 |
|                      |                                                                      |                                                    |
| ED_ATTEND_DATE       | NULL                                                                 |                                                    |
| Clean_IHD_VALUE      | 1                                                                    | NULL                                               |
| NULL                 | DIAGNOSIS_DATE                                                       | >=01/01/1907  and  <= audit end date (<31/03/2018) |
| and >= BIRTH_DATE    |                                                                      |                                                    |
| NULL                 | BIRTH_DATE                                                           | >=01/01/1907  and  <= audit end date (<31/03/2018) |
| and <=DIAGNOSIS_DATE |                                                                      |                                                    |
| BP_DATE              | >= audit start date (01/01/2017) and <= audit end date (<31/03/2018) | NULL                                               |
| HBA1C_mmol_Value     | Between 20 and 195                                                   | NULL                                               |
| HBA1C–_%_Value       | >= 3.979993595 and < 20                                              |                                                    |
|                      |                                                                      |                                                    |
| NULL                 |                                                                      |                                                    |
| BMI                  | >=12 and <=90                                                        | NULL                                               |
| SYSTOLIC_VALUE       | >=70 and <= 300                                                      | NULL                                               |
| DIASTOLIC_VALUE      | >= 20 and <= 150                                                     | NULL                                               |
| CREATININE_VALUE     | >=20 and <=1200                                                      | NULL                                               |
| CHOLESTEROL_VALUE    | >=1 and <=40                                                         | NULL                                               |

Certain records are excluded completely from the extracts submitted by GP practices and specialist services. Records are rejected for the following reasons: 

- A specialist service care record submitted with only NHS number and no 
corresponding primary care record. 
- A primary care record does not come from a GP practice which is eligible for the NDA. 
A list of eligible GP practices is obtained from NHS Digital corporate reference library, which is updated weekly using data from the Organisation Data Service (ODS) for England. We also sought feedback from Clinical Commissioning Group (CCG) contacts to ensure their GP practice list is up to date. NHS Wales Informatics Service (NWIS) supplied the list of eligible GP practices and associated LHB in Wales. 

 
Validation Data were automatically extracted from GP systems in England via system supplierdeveloped queries and GPES. Data submissions were initially checked and certified by GPES using test submissions. Internal validation checks were completed within the NDA team to ensure that data had been received from all eligible participating practices and there were no systematic issues with the data. Comparisons were made with GP practice and CCG level counts from previous years. One file was submitted for Wales by NHS Wales Informatics Service (NWIS) for the 2017-18 audit year and validated. Any data quality issues with the data are included alongside analysis to ensure users can interpret the results appropriately. For example, eye screening information was removed from the 2011-12 publication as the data was not deemed reliable, and the following statement was added to the report: 
"To improve alignment with NICE guidelines, a revised read code set of terms describing digital eye screening was used. This identified that variation in the use of terminology and its impact on the consistency of data extraction from electronic clinical records rendered it unreliable as a measure of this care process. The NHS Diabetic Eye Screening Programme (NHS DESP) records every digital eye screening and we believe that its records should now be used as the preferred measure for this annual care process. Presently this is reported only nationally." 
 
Production of Analysis database Most individuals within the NDA will have one record only which can be used to determine demographic and diagnostic information. Most records will come from primary care (GP extracts). The demographic fields include: 

- Age - Sex - Ethnicity - Lower Super Output Area (LSOA) - GP practice code 
 

- Diagnosis year - Diabetes type - Diagnosis of Learning Disability (LD) - Diagnosis of Serious Mental Illness (SMI) 
Diagnostic information includes dates and values for each of the care processes. Insulin pump data, submitted by participating specialist diabetes services, are included in the NDA from the 2015-16 audit year. Multiple readings data for people with diabetes are included in the NDA from the 2016-17 audit period. These data sets hold all a person's readings within the audit period for blood pressure, Body Mass Index (BMI), cholesterol and HbA1c (extracted from GP systems and dependant on GP practice participation). 

Drug data for people with diabetes are included in the NDA from the 2017-18 audit period. This data set holds all a person's drug prescriptions within the audit period for diabetesrelated medication: diabetes mellitus (including metformin), antihypertensive medication and statins (extracted from GP systems and dependant on GP practice participation). 

## Golden Record

Where data is received from more than one organisation a single record per person is created through the 'golden record' process. Information for the same person can be submitted by several organisations, for instance if the person has moved GP practices during the period, and/or attended one or more hospitalbased specialist services. For analysis purposes a single record is created for each person. 

The GP recording the most recent activity is assigned as the person's 'current' GP practice, as at the end of the audit period. Similarly, the specialist service recording the most recent activity is assigned as the person's 'current' hospital. If no GP or hospital data were received for a person then these data fields are left blank. Demographic information is taken from the current GP practice where this is valid. If this isn't the case, any other records are considered, and if they are not contradictory then this information is used. For example, if a current GP practice listed the sex as female, we would use this in the first instance. If the current practice submitted sex as 'unknown', other records for that person would be considered. If these records were a mixture of 'male' and 'female', sex would be left as 'unknown'. If the other records agreed that the only valid value for that person was 'female' then they would be assigned as 'female' in the data. The exception to this in demographics is the diabetes type. For this the value from specialist services is used where this is available, and not contradictory. Where it isn't available from hospital data, the diabetes type is taken from the current GP practice. If this is 'unknown' then other primary care records are considered, and the value used from these if they are not contradictory. For care processes the most recent date per person is used, with the lowest measurements taken that day assigned to the golden record. The record can therefore be made up of information from a mixture of GP practices and specialist services. 


## Analysis

Coverage The NDA provides a detailed picture of the clinical processes and care pathway for those diagnosed with diabetes. Some of the information collected as part of the audit is collected and published via the QOF. QOF is an aggregated return which provides information for nearly all GP practices in England. The QOF information is used within the report when discussing coverage. CCGs, Local Health Boards (LHBs), GP practices and adult outpatient specialist service units submit data about the care that is being delivered for people with diabetes in their organisation. This will include children that have been treated in an adult care setting, however it does not cover paediatric units. For the full picture on the paediatric care for children with diabetes, please refer to the National Paediatric Diabetes Audit (NPDA) which is conducted by the Royal College of Paediatrics and Child Health (RCPCH)1. The suite of NDA reports uses three separate cohorts of patients from England and Wales: 

- Primary care patients - These are patients who are registered with a participating GP 
practice.  
- Specialist service patients - These are patients that have a specialist service record. 
However, they may or may not be registered with a participating GP practice. Information is collected from hospitals and trusts via a bespoke data collection system. 
- All patients - Any patient from participating GP practices and specialist service units. 
The audit changed from an 'opt-out' to an 'opt-in' model from 2013-2014. A reduction in participation was observed in 2013-2014 and 2014-2015. Due to variation in participation caution should be taken when looking at local level analysis (CCGs and LHBs) for low participation years, and across time periods. Prior to 2013-2014 the approach to the audit was opt out, this meant that GP practices had to tell NHS Digital that they did not wish to participate; this was normally a small number of practices.  The drop in participation in 2012-2013 was expected due to the restructuring of the NHS in April 2013.  This organisational transition disrupted many well-established organisations and systems. A large amount of work was completed to improve participation in the audit the last two audit years, with a record 98.3% participation in 2017-18. 

| Audit year    | Total number of         | Number of    | National participation    |
|---------------|-------------------------|--------------|---------------------------|
| practices     | participating practices | rate         |                           |
| 2011-2012     | 8,549                   | 7,515        | 87.9%                     |
| 2012-2013     | 8,476                   | 5,991        | 70.7%                     |
| 2013-2014     | 8,232                   | 4,699        | 57.1%                     |
| 2014-2015     | 8,198                   | 4,696        | 57.3%                     |
| 2015-2016     | 8,021                   |              | 6,609                     |
| 2016-2017     | 7,742                   | 7,375        | 95.3%                     |
| 2017-2018     | 7,562                   | 7,435        | 98.3%                     |

Data period Data is collected covering a 15 month period between 1st January in the first year and 31st of March in the second year. 

 
Registrations  
This relates to the number of people registered with a GP or with a specialist service and covers the all patients cohort. It is expected that most patients should be registered with a GP, however there are instances where this is not the case. This may be because their registered GP practice has failed to participate.   
 
Definition of the Care Processes It is vital that people with diabetes receive all nine of the NICE recommended key health tests and measures. These tests help to monitor and manage diabetes, as well as to reduce the risk of complications such as stroke, heart disease and amputations. For each of the care processes the date fields are used to determine if the care process has been performed.  The most recent date is used in the audit year and these are matched back onto the relevant cohort of patients.  The denominator for the care processes are the total number of people aged 12 or over within the relevant cohort with the exception of HbA1c (where all patients are used). A care process can take place in any setting for example the blood test or the smoking review can be done in hospital or in the registered GP practice and it will still count towards the GP practice, CCG, LHB and the specialist service unit. The nine annual health checks for people with diabetes are: 
1. Blood pressure  
Blood Pressure is a measurement of the force of blood flow inside your arteries. Your blood pressure is stated as two figures, e.g.130/80. The first figure is known as the systolic pressure and relates to the pressure as the heart contracts and pushes blood through the arteries. The other figure is the diastolic pressure measured when the heart relaxes to refill with blood. A blood pressure measurement should be taken for all registered patients with diabetes at least once a year. This information is collected as part of the audit.  Records are only used within the care process completion indicator if there is a diastolic and systolic date and these are the same. Where a patient aged 12 or over has a diastolic and systolic date in the audit year at any organisation participating in the audit the care process is considered complete. 

2. Weight and BMI measurement  
Body Mass Index this should be recorded for all people with diabetes. This should be measured and recorded annually by the GP or specialist service unit. The date of this record is used to determine if the care process has been completed in the audit period. Where a patient aged 12 or over has a BMI date in the audit year at any organisation the care process is considered complete. 

For 2017-18, height and weight data were also collected for people with diabetes, alongside BMI, as one GP system supplier (Vision) did not have the facility to submit a calculated BMI. 

3. Serum creatinine test (creatinine is an indicator for renal function)  
This should be measured and recorded annually by the GP or specialist service unit. The date of this record is used to determine it the care process has been completed in the audit period. Where a patient aged 12 or over has a creatinine date in the audit year at any organisation the care process is considered complete. 

4. Urinary albumin test (or protein test to measure the kidney function) 
This should be measured and recorded annually by the GP or specialist service unit. The date of this record is used to determine if the care process has been completed in the audit period. Where a patient aged 12 or over has an albumin date in the audit year at any organisation the care process is considered complete. 

5. Cholesterol (Triglycerides are another type of fat in the blood) 
If you have raised cholesterol and raised triglycerides you have an increased risk of Cardio Vascular Disease (CVD).  This should be measured and recorded annually by the GP or specialist service unit. The date of this record is used to determine it the care process has been completed in the audit period. Where a patient aged 12 or over has a cholesterol date in the audit year at any organisation the care process is considered complete. 

6. Eye check (retinopathy screening)  
Diabetic retinopathy is a common complication of diabetes. It occurs when high blood sugar levels damage the cells at the back of the eye, known as the retina.  Patients with diabetes should be offered screening on an annual basis. Unfortunately, the variation in the use of terminology and its impact on the consistency of data extraction from electronic clinical records rendered this element of the data unreliable as a measure of this care process. The NHS Diabetic Eye Screening Programme (NHS DESP) records every digital eye screening and we believe that its records should now be used as the preferred measure for this annual care process. 

7. Foot check Patients with diabetes should have an annual foot check, as diabetes can limit the blood supply to your feet and cause a loss of feeling. This can mean foot injuries do not heal well. A patient is more likely to have a limb amputated when they have diabetes.  Where a patient aged 12 or over has a foot exam date in the audit year at any organisation the care process is considered complete. 

8. Smoking status  
There has been a change to the definition of the smoking care process in 2017-18. This was to bring the care process into line with updated Quality and Outcomes Framework (QOF) rules. The care process is considered complete where: 

a) a patient aged 12 or over has a smoking status date in the audit year or; b) a patient has a smoking status of "Never smoked" dated prior to the audit period, 
but after the patient was 25 years old. 
9. Blood test (HbA1c - blood glucose levels)   
This indicates your blood glucose levels for the previous two to three months. The HbA1c measures the amount of glucose that is being carried by the red blood cells in the body. This care process is appropriate for all ages. Where a patient of any age has an HbA1c date in the audit period at any organisation the care process is considered complete. 

 
All 8 Care Processes  
The person must have received all eight care processes (excluding eye screening) where appropriate. The requirement is that everyone has their HbA1C measured annually, and everyone aged 12 or over should have all eight care processes annually. So, an 11-year-old will only be required to have their HbA1c value tested and so if this has been done they will have achieved the all care process element, However, an adult aged 25 will have to have received all eight different care processes to have achieved the all eight care processes. 

 
Definition of the Treatment Targets There has been a change to the NICE Guidelines, which has resulted in new treatment targets for the NDA. The new guidelines relate to the prescription of statins for primary and secondary prevention of cardiovascular disease. For 2017-18, the new treatment targets have been reported alongside the 'old' targets. The 'old' targets relate to three of the care processes; blood tests (HbA1c - blood glucose levels), cholesterol and blood pressure. For a patient to appear in the denominator group for each of the targets, they need to have the date and valid result recorded for the care process. The most recent date is used in each audit year and the best result for the care process is taken if two tests were completed on the same day. These are matched back on to the relevant cohort of patients. A care process, and therefore the target result, are taken from any setting and it will count towards the GP practice/CCG and specialist service unit targets. 

## 'Old' Treatment Targets The Seven Targets For These Three Care Processes For People With Diabetes Are:

 
1. Blood test (HbA1c - blood glucose levels)  
This test is vital to monitoring the long-term glucose levels within the patients' blood; the aim should be to keep these at normal levels. The most recent result is taken for use within the calculation and if there were two tests completed on the same day the lowest value is used. There are three cumulative level targets: 

1. The HbA1c value is less than 48 mmol/mol (<48) 2. The HbA1c value is less than or equal to 58 mmol/mol (≤58) 3. The HbA1c value is less than or equal to 86 mmol/mol (≤86) 
 
2. Cholesterol  
If you have raised cholesterol and raised triglycerides you have an increased risk of CVD. This target measures the cholesterol levels of patients with diabetes. The most recent result is taken for use within the calculation and if there were two tests completed on the same day the lowest value is used. 

There are two cumulative level targets: 

1. The cholesterol value is less than to 4 mmol/l (<4) 2. The cholesterol value is less than to 5 mmol/l (<5) 
 
3. Blood Pressure  
This is a flat target that the patient's blood pressure (BP) should be less than or equal to 140/80.  Both systolic and diastolic pressures must be within their respective limits for this treatment target to be met. The most recent result is taken for use within the calculation and if there were two tests completed on the same day the lowest value is used. 

Old combined 'All three treatment targets'   
The person must achieve HbA1c ≤ 58 mmol/mol, cholesterol < 5 mmol/l and blood pressure <=140/80 to have achieved the 'All three treatment target measure'. For children under the age of 12 years, meeting 'All three treatment targets' is defined as meeting the HbA1c requirement only, as other treatment targets are not recommended in the NICE guidelines for this age group. 

## 'New' Treatment Targets

Primary prevention of cardiovascular disease (CVD) 
This shows people aged 40 to 80 years with no history of CVD who are recorded as being prescribed statins medication at any point in the audit period. No history of CVD means the individual has had no hospital admission record mentioning angina, stroke, myocardial infarction (MI) or heart failure in the ten years up to 2017-18 and is not recorded in NDA as having ischemic heart disease (IHD). 

## Secondary Prevention Of Cvd This Shows People Of Any Age With A Diagnosis Of Cvd Or Ihd Who Are Recorded As Being Prescribed Statins Medication At Any Point In The Audit Period. Cvd - Medication Combined Measure This Shows People Without Cvd Or Ihd Between The Age Of 40 And 80 Years Or With Cvd/Ihd At Any Age That Are Recorded As Being Prescribed Statins Medication At Any Point In The Audit Period.

 
New combined 'All three treatment targets'  
This shows people achieving an HbA1c value ≤ 58mmol/mol, blood pressure (BP) ≤140/80 and, if their use is indicated, being prescribed statins. For children under the age of 12 years, meeting 'All three treatment targets' is defined as meeting the HbA1c requirement only, as other treatment targets are not recommended in the NICE guidelines for this age group. For people who prescription of statins is not indicated, meeting 'All three treatment targets' is defined as meeting the HbA1c and blood pressure requirement only.  

Standardisation Indirect standardisation was used to compare the number of people completing care processes with the number expected to. Logistic regression models were built to estimate expected numbers of people completing care processes for each care provider and commission. The models were constructed for each audit year for the whole of England and Wales. The observed numbers of people completing care processes or achieving treatment targets were compared to these expected numbers.  A 'banding score' was assigned to each care provider or commission according to whether the observation was above, in-line-with or below expectation.  Noting that when the expected number of people with diabetes completing a care process is small a banding of 'Insufficient data' is shown. However, banding is not a measure of quality of care and a higher or lower than expected number of people completing care processes should not immediately be interpreted as indicating poor or good performance. The banding should not be treated as an absolute assessment of performance and cannot be used to compare directly performance between GPs and CCGs/LHBs, but rather as a tool to aid local investigation. Demographic variables, such as ethnicity, were included in the models to determine, as an example: if a younger male of Asian ethnicity is more likely to have had a particular outcome compared to an older female of white ethnicity. The models allow each variable to be considered independently by controlling for the effects of other, sometimes related factors. When modelling data, the aim is to produce a model which both satisfies certain statistical criteria and maintains a connection with the real-world understanding of the behaviours we are trying to explain. The ability of the model to explain the variation seen in the data and to correctly predict outcomes is assessed using the c-statistic. The values for this measure range from 0.5 to 1.0. A value of 0.5 indicates that the model is no better than chance at predicting care process completion and a value of 1.0 indicates that the model perfectly identifies those who will and those who will not complete a care process. Models are typically considered reasonable when the c-statistic is higher than 0.7 and strong when the c-statistic exceeds 0.8.  
 Explanatory variables were selected by consultation with our clinical lead and advisory group. However, not all variables made significant improvements to the model and, where this was the case, these were removed. Variables included in the care process models were:  

- Age band - Ethnic group - IMD quintile group (a measure of deprivation based on a patient's postcode) - Sex - Smoking status - BMI group - Duration of diagnosed diabetes - Whether or not the person was prescribed statins 
The outputs from the logistic regression models are presented in the supporting tables to allow users to review the quality of the models derived and these tables include the c statistic associated with each model. Banding has been investigated for treatment target achievement results but was judged not to be appropriate. This is because the statistical models did not predict with enough certainty whether an individual was likely to achieve the treatment target. Accordingly, it is likely that achievement of treatment targets is largely driven by factors other than the patient characteristics captured in the NDA.  
 
Structured Education  
Care process completion and treatment target achievement is calculated for each audit year using data from that audit year collection. Structured education information is calculated on a different basis, as it looks at people diagnosed within a calendar year and whether education was offered/attended in the 12 months following diagnosis. The diagnosis dates for each person are taken from the most recent audit year's data collection (i.e. NDA 2017-18) and reported for each calendar year (i.e. 2015, 2016). As new NDA data is received, the structured education figures are re-calculated to consider audit participation increases and attempts at a local level to retrospectively update offered/attended data in GP systems. The latest structured education data reported in the NDA 2017-18 publication are for people diagnosed in calendar year 2016. This is because if a person is diagnosed in 2017, they would need a full 12mth opportunity to be offered/attend structured education, however the NDA, at that point in time, would only hold data up to March 2018. As an example, a person diagnosed in November 2017 may attend structured education in July 2018 (within 12mths of diagnosis) but the attendance would not yet be recorded in NDA data.  
 
Learning Disabilities  
Learning disability (LD) information was first collected during the 2015-16 audit where it was found that the LD population had a substantially different age and sex profile compared to the NDA population as a whole.  
Therefore, to permit a fair comparison, results for the LD population were directly standardised by age and sex to the whole NDA population. Comparison to the whole NDA population, of which only a very small proportion have LD, is consistent with other sections of the report. Individuals with a learning disability diagnosis were identified using the codes listed in the NDA Primary Care Extract Specification. 


## Disclosure Control Disclosure Control Has Been Applied To Mitigate The Risk Of Patient Identification.

In years prior to 2016-17, the data for practices was removed from the publication where the practice list size was less than 1,000. Data for additional practices was removed where differencing would reveal suppressed data. This was compliant with the Anonymisation Standard for Publishing Health and Social Care Data, but data for all practices that had submitted data was not available. In addition, providing data for CCG, LHBs, specialist services or GP practices broken down by demographics such as age required additional ad-hoc suppression that was time consuming and meant much of this information was not practical to produce. From 2016-17, a new suppression method was implemented. Zeros are reported, and all numbers are rounded to the nearest 5, unless the number is 1 to 7, in which case it is rounded to '5'. This allows for more granular data to be made available, and also for data for all GP practices to be made available. Rounded numbers are used to calculate rates such as care process completion and treatment target achievement. At CCG/LHB level and above this makes virtually no difference to the resultant rates. At GP level, where the numbers are small, this rounding can have a relatively large impact on the resultant rates, but where numbers are small, rates are volatile and should already be treated with caution.