Entering Data for Analysis

Annie HerbertMedical Statistician

Research & Development Support UnitSalford Royal (Hope) Hospitals NHS Foundation Trust

annie.herbert@manchester.ac.uk 0161 20 64567

Sept 2009 (v.2)

Timetable

Time Task

60 mins

Presentation

(Trust Library

Meeting Room)

20 mins Coffee Break

90 minsPractical Tasks

(Trust Library IT Room)

Outline

ENTERING DATA:• Data Format• Missing Data• Anonymity

STATSDIRECT:• Layout• Re-coding Variables• Transforming Variables• Categorising Variables

Spreadsheets

Things to remember:

Variable names:• Keep short• SPSS : No spaces, may use underscore,

e.g. ‘dose_1’

Categories:• Give each category a numerical code

e.g. medical = 1, surgical = 2, other = 3

Format

• No mixture of variables per column

e.g. blood pressure, rather than 180/60 have a column for diastolic and a column for systolic

• Format columns for dates, money, etc.

e.g. two decimal places for money

• Make sure that the program recognises whether the variables are continuous or categorical

Units• Do not enter with data

• May put in variable name e.g. ‘weight_kg’

• No mixtures of units per column

e.g. in 100’s and then switch to in 1000’s

Group Identifiers

Missing Data

• StatsDirect : use ‘*’

• SPSS : use invalid value, e.g. ‘999’ for age

• If not using ‘*’, make sure that the package identifies such values as missing and not a real observation

Anonymity

• Each patient/unit in the study should be given a unique ID code

• Nobody outside the care-team should be able to identify the patient (e.g. statistician)

• Violation of Data Protection Act if carrying identifiable data on disc/CD/laptop

Transforming Data

Definition: Making a new variable by applying a formula to one (or more) existing variables.

Uses:• Calculating a new variable,e.g. the difference between pain at 6 months and pain

at baseline.

• Changing units,e.g. turning days into weeks.

• Taking natural logarithms in attempting to make data symmetric.

Recoding Data

Definition: Changing existing numbers into different ones

Uses:• Changing text to numbers,

e.g., Female = 0, Male = 1

• Changing a coding system,e.g., Female = 1 -> 0, Male = 2 -> 1

• Combining categories, e.g., Strongly Disagree = 0 & Disagree = 1

-> Negative Response = 0 Agree = 2 & Strongly Agree = 3

-> Positive Response = 1

Categorising Data

Definition: Converting a continuous variable to a categorical one.

E.g., Less than 30 years = 130-50 years = 251 years and over = 3

Special Types of Study

Questionnaires:• One row per questionnaire• One or many columns per question,

depending on number of answer options

Repeated Measures:• One row per person• One column per measure• Seek statistical advice

Available PackagesPackage Advantages Disadvantages

SPSS

• Journal-quality graphs

• Difficult to use at first• Expensive• Need license

StatsDirect• Easy to use • Cheap

• Need license

Excel

• Comes with Microsoft Office• Good for data entry

• Not good for statistical analyses

StatsDirect

Data can be entered and stored in a ‘Workbook’:

Plots, tables and analysescan be shown in a ‘Report’:

You can work between many workbooks and reports at a time:

Analyses are carried out using the menu headings:

A prompt will appear at each stage:

You’ll be asked to select the relevant data:

Transforming Variables (1):

1)2)

3) 4)

Transforming Variables (2):

5)

6)

Recoding Variables (1):

1)

2)

3)

Recoding Variables (2):

4)

5) 6)

Categorising Variables (1):

1)

2)

3)

Categorising Variables (2):

4)i.

5)

4)ii.

4)iii.