Spreadsheet Engineering

• Builders use blueprints or plans– Without plans structures will fail to be effective

• Advanced planning in any sort of design can speed up implementation

• Spreadsheets are no different from other arenas of design– Engineering and advanced planning aids

effectiveness

5 - 1

Spreadsheet Design

• An efficient process minimizes time spent• An effective process yields results that meet

users’ requirements• Good design helps analysts spend the majority

of their effort improving decisions, rather than building and fixing models

5 - 2

The Phases of Spreadsheet Modeling

• Designing• Building • Testing

5 - 3

Designing a Spreadsheet

• Plan• Modularize• Start small• Design for use• Keep it simple• Design for communication• Document important data and formulas

5 - 4

Plan

• “Measure twice, cut once”• Will decrease time spent correcting mistakes• Turn computer off and think before beginning• Begin with a sketch– Physical layout of major elements– Rough indication of calculation flow

• Anticipate model’s ultimate uses

5 - 5

Modularize

• Group like items and separate unlike items• Separate– Data– Decision variables– Outcome measures– Detailed calculations

• Influence diagrams aid with this design• Formulas should generally reference cells above and

to the left

5 - 6

Start Small

• Sketch full design but do not build all at once• Isolate one module then build and test that

module • Local mistakes much easier to detect than

when part of the global model

5 - 7

Design for Use

• Anticipate who will use spreadsheet– What type of questions will be asked?

• Make it easy to change common parameters• Make it easy to find key outputs– Group in one place

• Include graphs of outputs• Record numerical values of base case outputs

5 - 8

Keep It Simple

• Complex spreadsheets:– Require more time and effort to build– Are much more difficult to debug

• Keep formulas short– Decompose complex calculations into

intermediate steps

5 - 9

Design for Communication

• Spreadsheets’ lives are often longer than expected

• Use visual cues that reinforce model’s logic– Use informative labels– Use blank spaces– Use outlines, color, bold fonts, as appropriate

• Split windows can aid in viewing

5 - 10

Document Important Data and Formulas

• Record source for important parameters• Explain important formulas• Use Cell Comments to describe cell contents• Consider a separate module to list

assumptions

5 - 11

Cell Comments• Insert – Comment to add documentation to a cell– Comment & indicator – permanently display comment– Indicator – red triangle indicates comment, display when

cursor in cell– None – neither comment nor indicator visible

5 - 12

Workbook Design

• Use separate sheets to group similar kinds of information

• Design workbooks for ease of navigation

5 - 13

Use Separate Sheets to Group Similar Kinds of Information

• Makes model easier for outsider to use• Isolate technical details of model• Bring assumptions and results to the fore• Users should only interact with a few, easily

recognized, sheets

5 - 14

Design Workbooks for Ease of Navigation

• Any form of structural help for users is beneficial

• Use revealing names for individual sheets– Double-click on name tab at bottom of

spreadsheet to edit name

5 - 15

Building a SpreadsheetFollow a planBuild one module at a timePredict the outcome of each formula Copy and paste formulas carefullyUse relative and absolute addressing to simplify

copyingUse the Function Wizard to ensure correct syntaxUse range names to make formulas easy to readUse dummy input data to make errors stand out

5 - 16

Testing a Spreadsheet

• Check that numerical results look plausible• Check that formulas are correct• Test that model performance is plausible

5 - 17

Check That Numerical Results Look Plausible

• Make rough estimates• Check with a calculator• Test extreme cases

5 - 18

Check That Formulas Are Correct

• Check manually• Display individual cell references• Display all formulas• Use the auditing tools• Use error checking

5 - 19

Summary

• Spreadsheets deserve careful engineering– Most spreadsheets contain errors– Users are over confident about their models

• Rules for spreadsheet modeling– Designing a spreadsheet– Designing a workbook– Building a spreadsheet– Testing a spreadsheet

5 - 20