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Problem Management Escalation Spreadsheet - PowerPoint

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					           Spreadsheet Design Concepts




         Simon Murphy
         simon.murphy@codematic.net
         Developer – Codematic Ltd

  UK XL User
Conference 2006
                  Spreadsheet background
   • Up to 200 Mb size
   • Up to 1 Million formulas
   • 1-10,000 unique formulas
   • 5-10,000 lines of VBA
   • £Billions in values
   • Often linked to other technologies such as OLAP, ADO,
     COM or .net etc.
   • Finance, Banking and Sales and Marketing areas
   • Development cost up to $1M
   • Active member of Eusprig – European Spreadsheet Risk
     group – dedicated to raising awareness of dangers and
     error rates in commercial spreadsheets www.eusprig.org

  UK XL User
Conference 2006
                            Agenda
   •   Fundamental Imperative
   •   Security
   •   Software Development Lifecycle
   •   Design
        –   Technology choice
        –   Inputs
        –   Data
        –   Logic/Formulas
        –   Documentation
   • Summary
   • Any Questions

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Conference 2006
                  Fundamental Imperative
   • Manage complexity (McConnell)
   • Solution complexity grows at 4 x the rate of
     problem complexity. (Glass)
   • [Things] should be as simple as they can be, but
     no simpler (Einstein)
   • K.I.S.S.
   • This principle should drive all other work.
   • Easier to build, easier to test, easier to
     document, easier to use, etc… [No conflict]
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Conference 2006
                                      Security
    •   Is everybody‟s concern
    •   Spreadsheets can be used as a staging board for privilege escalation (with
        your login details!)
    •   Consider SD3 +C
         – Secure by
                  •   Design
                  •   Default
                  •   Deployment
                  •   Communication
    •   Threat Modelling – Assets, Threats
    •   Threat Types – STRIDE
         – (Spoofing, Tampering, Repudiation, Information Disclosure, Denial of Service,
           Elevation of Privilege)
    •   Threats – rate with DREAD
         – (Damage potential, Reproducibility, Exploitability, Affected Users,
           Discoverability)
    •   Spreadsheets (all flavours) are fairly insecure
         – Compiled UDFs (.net, COM, XLL) and Database servers can help
    •   Set macro security to high and use code signing certificates.
    •   See Microsoft MOC 2840A – Implementing security for more info.

  UK XL User
Conference 2006
   Software Development Lifecycle
 • Systems Development lifecycle
       –   Requirements,
       –   Analysis,
       –   Logical Design,
       –   <Technology Choice>,
       –   Physical Design,
       –   Construction,
       –   Test,
       –   Release,
       –   Maintain.
       –   In some shape or form.

 • Spreadsheet Development lifecycle – “Oh! I
     need a model” – clickety-click, reasonableness check,
     release, (Test & Maintain in live environment).
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Conference 2006
                  Big Design Issues
  •   Project Scope
  •   Business needs
  •   Security
  •   Performance
  •   Maintainability
  •   Extensibility
  •   Availability
  •   Scalability
  •   Human factors
  •   Integration
  •   Methodologies

  UK XL User
Conference 2006
                  Design – Excel or not
  • Excel/VBA is often not technically the best
    solution
        – Databases better for large volumes of data
        – Compiled languages better for security
        – Spreadsheets are a 2 dimensional tool where most
          business problems are 5+ dimensions
  • It is good when considering
        –   Initial speed of development
        –   Cost of initial development
        –   Current skill sets
        –   Simple initial deployment

Note: If you need to restrict Excel functionality you may be better outside Excel
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  Design – 2 Questions, 2 Approaches
  • 2 Questions
        – What will happen when things go right?
        – What will happen when things go wrong?
  • 2 Approaches:
        – What will the system do?
        – What real world objects am I modelling?




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Conference 2006
                  Assuming Spreadsheet
   • Design
        –   N-Tier
        –   Defensive Designs
        –   Inputs
        –   Data
        –   Layout
        –   Formulas
        –   VBA
        –   Extending Excel
        –   Documentation
  UK XL User
Conference 2006
                          N-Tier
   • Basic version is 3 tier
        – Data layer
        – Logic (or analysis) layer
        – Presentation layer
   • Each of these may be further broken out
     (into N tiers)
   • Easily implemented in worksheets
   • Suitable for most non trivial workbooks
   • Powerful and flexible but adds weight
  UK XL User
Conference 2006
                           Basic design (N-tier)
      Model                           Support structure



                  Inputs                  Assumptions




          Reference Data               WorkbookStructure




                   Logic                 Accountability




                  Reports               Revision history   Demo


  UK XL User
Conference 2006
                   Defensive Designs
  •   How will I test this?
  •   Clearly identify Input areas
  •   Group items that need updating with similar frequency
  •   Place formulas where they are safe from accidents
  •   Don‟t use sheet protection, its weak, annoying, counter productive,
      and reduces peoples ability to check and understand your logic (it
      creates more problems than it solves),(Trust – but check) Use a
      compiled component if security is a genuine concern. Demo
  •   Use cross checks
  •   Use a (simple) consistent convention to separate items that should
      be treated differently (inputs/formulas or actual/forecast)
  •   Use blanks cells around different blocks (enables „Current Region‟)
  •   Consider using a status sheet with summary error reports on it
  •   Consider support structure – Index, assumptions, accountability.
  •   Consider personal and tool comfort zone
  •   Place check controls where they will be noticed


  UK XL User
Conference 2006
                          Inputs
 • Never trust inputs
 • Always test for correctness as soon as possible
 • Limit inputs wherever possible (eg option buttons, or drop
   downs)
 • Use data validation but be aware of its limits demo
 • Help users by clearly identifying what is needed
 • Give clear feedback on errors or problems
 • Minimise input – Let Excel do the work
 • Have validation formulas nearby
 • If test is ok say “ok” (or “Row/Column check ok”) not “”
 • Prefer Text in cells rather than comments


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Conference 2006
                   Database (Relational)
   • Useful for flexible and efficient storage and updating
   • Identify Entities (nouns), Attributes (adjectives) and
     relationships (verbs) in problem statement
   • 2 approaches
        – Top down (identify entities then attributes)
        – Bottom up (group attributes to describe entities)
   • Data normalisation
        – Basically keep related items together
        – Provides design flexibility
        – 3rd Normal Form (TNF/3NF)
              • All items depend on the key (a unique identifier), the whole key
                and nothing but the key.
   • More useful at the transaction processing end rather
     than analysis and reporting. (eg sales recording)
   • Analysis performance can be poor
   • PL demo
  UK XL User
Conference 2006
                         Database (OLAP)
 • Useful for flexible/powerful reporting.
 • The most useful concept of Dimensionality – the number of ways to
   describe something
 • In a spreadsheet it‟s the row, column, worksheet, workbook, and
   maybe directory.
 • Very useful for things that may ordinarily be missed
 • How to describe the numbers (dimensionality)
       – Eg Inflation <Time?, Country?, Cost Type?>
       – Eg. P&L value <P&L line description, Time, Business unit, value type
         (actual, budget etc), currency, Company etc>
       – Useful for layout – time in cols or time in sheets?
 • Hierarchies are ways of adding up dimension elements
       –   There may be more than one hierarchy for any given dimension
       –   Eg time
       –   day > week > quarter > financial year
       –   day > calendar month > calendar year
 • Reporting performance can be excellent as many calculations pre-
   aggregated

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Conference 2006
    Spreadsheet eg. of Dimensionality




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Conference 2006
   Spreadsheet eg. of Dimensionality 2



                                338 is
                                Period 8 2005
                                Actual Figure
                                For
                                Employment
                                Costs
                                For the North
                                For Large Co




  UK XL User
Conference 2006
                  Pivot Tables
   • Probably the most important feature in
     spreadsheets
   • A superb way to manage complexity
   • Most users can‟t access them because their data
     is already half pivoted.
   • Repeated blocks are a strong hint to use pivots
   • Pivot source data should have 1 one column
     with numbers in (slightly simplified)
   • Demo
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       Software Development Principles
   •   Modularisation
   •   Cohesion
   •   Coupling
   •   Fan in / Fan out




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Conference 2006
                                   Modularisation
   •   Basic idea – to break down complexity into understandable chunks (note
       Millers Theorem (7+/-2))
   •   Advantages
        –    Simplifies and adds analysis layers
        –    Adds flexibility
        –    Improves robustness
        –    Reduces dependencies
        –    Improves testability
   •   Disadvantages/Limitations
        – All Cells can be read from everywhere
        – Can add redundancy
        – Not really applicable for very small models
   •   Use of a block of cells for one (single) task


                                                        Volume Summary
          Volume Calcs
         (eg Geographic)                                  Sales Price

            Total Volumes                                Total Revenue

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             Worksheet modularisation




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Conference 2006
          Worksheet modularisation 2




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     Worksheet modularisation 3 (bad)




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Conference 2006
                        Modular blocks




A „block‟ is a area of cells surrounded by empty cells, that performs some analysis

  UK XL User
Conference 2006
                       Cohesion
    • How interrelated a unit is
    • High cohesion means all elements are highly
      interrelated – this is good, it aids understanding
      and reduces range of influence
    • Low cohesion makes things harder to
      understand like random letters
    • Cohesion is like well normalised data – but also
      considers what the unit does
    • Cohesion test…


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                  Cohesion 2
   • A<kfj\s =]‟#\kdjw487



   • Give me all your chocolate




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                   Cohesion 3
   • Who can remember the first string?



   • The second?




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                  Cohesion 4
   • Keep different shaped data blocks apart
   • Either diagonally or on separate sheets
   • Try to make all formulas in a block similar




  UK XL User
Conference 2006
                  Poor Cohesion




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Conference 2006
                  Poor Cohesion 2




  UK XL User
Conference 2006
                   Coupling
  • How strongly 2 separate elements depend
    on each other
  • Low coupling is better, especially through
    clearly defined interfaces
  • High coupling often means hidden
    dependencies which generally leads to
    incorrect modifications (side effects)
  • Example: hard coded cell addresses in
    VBA code

  UK XL User
Conference 2006
                   Fan in / fan out
   • Low fan in – a „unit‟ (cell/worksheet/VBA
     routine) depends on only a few other units
        – Good because it minimises dependencies and reduces
          complexity
   • High fan out – a „unit‟ (cell/worksheet/VBA
     routine) is used by many others
        – Good because it minimises duplication
   • Example: putting VAT (Sales tax) rates in their
     own cells, and referring there in calculations
   • Example: calculating an offset once and using
     the result many times

  UK XL User
Conference 2006
                      Logic/Formulas
   •   Sketch design on a whiteboard first
   •   Don‟t use IF(ISERROR(, be specific demo
   •   Put expected part first in Ifs
   •   Don‟t start in A1 – Give yourself room to manoeuvre. Try D10 – hide
       the unused.
   •   Line sheets up on first data cell rather than headers
   •   Don‟t hide rows and columns for visual effect, use a separate sheet
       if required
   •   Use Goal seek and or VBA rather than circular references
   •   Avoid more than 2 or 3 levels of formula nesting – break it out
       across several cells.
   •   Use tools – many pay for themselves on first use. But don‟t totally
       rely on them.
   •   Place totals above and to the left of details (more flexible and
       robust eg links, filtering)
   •   Build for testability – how to test you did what you meant to do

  UK XL User
Conference 2006
                               VBA Connection
 •   VBA UDFs should get all range info via parameters
 •   Where (non UDF) VBA uses worksheet ranges, these should almost certainly
     be named ranges.
      – Sheet1.[Inflation].value = 0.02, rather than
      – Sheet1.[C5].value = 0.02, which may become invalid if rows or columns
         are inserted or deleted
      – strInflationRange = “C5” is just as bad – it creates a hidden dependency
         that must be manually updated when worksheet changes (poor coupling)
 •   Variable and routine names:
       –   Use very meaningful names (8-30 chars length)
       –   Use a simple naming convention (matched pairs)
       –   Use scope prefixes (g, m), and minimise it
       –   Don‟t have to use data type variable prefixes (s, str, l, lng etc) (see .net advice)
       –   avoid abbreviations
 •   Don‟t use code comments demo make code ultra clear instead
 •   Option Explicit on
 •   Avoid Application.Run (non VBA – breaks error management)
 •   routines 1 screen long max
 •   3-4 levels of nesting max
 •   Use source control
  UK XL User
Conference 2006
                  Extending Excel
   • Excel is powerful not perfect
   • Leverage benefits whilst managing weaknesses
     by using complimentary technologies
   • Data: VBA, ADO, ODBC, OLAP, .net, COM, Info
     Bridge, XML, DDE, Web Queries, SOX Solution
     Accelerator
   • Logic: VBA, COM, .net, xll, Pivots, Filters
   • Search the web for vast array of free or cheap
     tips and tools.

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Conference 2006
                   Documentation
• If you design for simplicity only very minor additional
  documentation is needed
• Design and build the user Interface with the Users needs
  primary, the documentation will automatically be there
• Design and build the business logic parts with the
  maintainers needs primary, the main documentation will
  automatically be there, expand as required.
• Integral tests should explicitly clarify intent.
• Reports should contain enough description to be
  meaningful.
• External documentation is almost always so out of date its
  worse than useless.
• Excessive documentation is too hard to plod through
• Poorly targeted documentation is pointless
• Working software is more useful than documentation
• Sometimes documentation is important
  UK XL User
Conference 2006
                    Summary
   • Manage complexity
   • Consider security carefully
   • Excel/VBA not the best tool for everything
   • Be defensive, especially with inputs
   • Understand your data
   • Aim for cohesive models with low coupling
   • Use names to connect VBA to worksheets
   • Use complimentary technologies where
     appropriate
   • Manage documentation

  UK XL User
Conference 2006
                      Questions?
   • simon.murphy@codematic.net
        – Spreadsheet consulting, reviewing,
          maintaining, rescuing, migrating, add-in
          development etc.
        – Staff coaching, mentoring and training
   • Websites
        – www.codematic.net
        – www.xlanalyst.co.uk

  UK XL User
Conference 2006

				
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