Trusted by Scottish Government
NHS 24
Thoughtworks
Back to Blog
Data Governance05 April 202610 min read

Spreadsheets Are Killing Your Forecasts: When Excel Becomes a Liability

Excel is the most dangerous software in your organisation. Not because it's bad - but because it's so good that it becomes the source of truth for decisions it was never designed to support.

The Spreadsheet Trap

It always starts innocently. Someone needs to track a budget, so they create a spreadsheet. It works well. Others ask for access. Soon it's "the budget file" that everyone references.

Fast forward two years: that spreadsheet now has 47 tabs, links to 12 other files, macros that nobody understands, and three people who each have their own "latest version."

The Scale of the Problem

Studies suggest 88% of spreadsheets contain errors. In a 2013 analysis of 100 spreadsheets, 94% contained errors, with an average of 5 errors per sheet. Most go undetected until they cause real damage.

Why Spreadsheets Fail at Scale

1. No Single Source of Truth

"Budget_Final_v3_FINAL_JohnEdits_APPROVED.xlsx" - sound familiar? When multiple people maintain copies, nobody knows which one is current. Every meeting starts with "which version are we looking at?"

2. No Audit Trail

Someone changed a number. When? Who? Why? Without version control and change tracking, you're trusting that whoever edited last knew what they were doing. And that they didn't accidentally delete a formula.

3. Fragile Formulas

Insert a row and break a SUM. Copy a cell and lose a reference. The logic in spreadsheets is invisible and brittle. One wrong move can silently corrupt calculations that propagate through dozens of dependent cells.

4. Manual Updates

Every month, someone spends two days pulling data from five systems, copying it into the master spreadsheet, and reconciling discrepancies. By the time it's done, the data is already a week old.

5. Knowledge Concentration

"Ask Sarah, she built the spreadsheet." When the creator leaves, the organisation loses the only person who understands how it works. The file becomes a black box that everyone depends on but nobody can maintain.

Real-World Failures

Spreadsheet errors have caused some spectacular failures:

  • JP Morgan's London Whale: A copy-paste error in a risk model spreadsheet contributed to $6 billion in losses
  • MI5 Bug: A formatting error led to the surveillance of over 1,000 wrong phone numbers
  • Barclays Acquisition: Hidden rows in a spreadsheet accidentally included 179 contracts Barclays didn't intend to acquire

These are the failures that made headlines. Most spreadsheet errors never get discovered - they just quietly produce wrong decisions.

When Spreadsheets Make Sense

Spreadsheets aren't the enemy. They're brilliant for:

  • Quick, one-off analysis
  • Personal data exploration
  • Prototyping before building something proper
  • Simple lists and tracking for small teams

They break down when they become:

  • The source of truth for the organisation
  • Shared and edited by multiple people
  • The basis for significant financial decisions
  • Something that needs an audit trail

The Migration Challenge

Everyone knows they should move off critical spreadsheets. Few do. Why?

Common Objections

  • "It would take too long" - Often true for complex replacements. But the cost of not migrating compounds every month.
  • "This one's different" - Every critical spreadsheet started as a simple file that "just grew."
  • "We'd lose flexibility" - Real systems can be flexible too, with the added benefit of not breaking silently.

A Practical Path Forward

You don't need to eliminate all spreadsheets overnight. Start with:

  1. Identify critical files: Which spreadsheets drive significant decisions?
  2. Assess risk: What happens if this file has an error? How would you know?
  3. Connect, don't replace: Often you can keep the spreadsheet but feed it from a system of record
  4. Add governance: Track changes, validate formulas, establish ownership

How FireBreak Helps

FireBreak can connect to your spreadsheets - pulling data in without requiring you to abandon what works. But it also provides the governance layer spreadsheets lack: validation rules that flag anomalies, audit trails that track every change, and a single source of truth that everyone can trust. Keep your flexibility, lose the risk.