Really, that statistic needs to be repeated: 95% of spreadsheets have errors.

This is not just a random, made-up statistic. Extensive research into spreadsheets has consistently found that almost all spreadsheets have errors.

Large spreadsheets are more likely to have errors, meaning that they're almost certainly wrong.

## Research and experience agree

Our assertion that 95% of spreadsheets have errors is based on:

## How can a 95% error rate be true?

It seems surprising that a spreadsheet almost certainly has errors. So, how does that happen?

Most people think that they are very accurate when doing most activities – and they're right. Research has shown that, for a wide range of cognitive tasks, humans make an error in only 1% to 5% of tasks.

For example, typical error rates for simple, non-trivial activities are:

• Type a short number: 1.0% (per number).
• Grammatical errors: 1.1% (of words).
• Simple arithmetic: 2.0% (of calculations).
• Software development: 3.7% (per line of code).
• Type 10 digits: 5.0% (per number).

## The spreadsheet Cell Error Rate (CER)

Experiments in spreadsheet development have seen similar rates, with errors in 1% to 5% of cells – this is called the "Cell Error Rate" (CER).

Although a CER of 1% to 5% seems low, the cascading nature of spreadsheet calculations means that errors accumulate through the calculations down to the bottom-line results.

Most spreadsheets have hundreds or thousands of formulae. Even with a small probability of error in each formula, the accumulated probability that bottom-line results have errors is bound to be high. Even for a small spreadsheet, the accumulated probability of error tends towards 100%.

The probability that a spreadsheet has at least one error is shown in the following chart.

The chart assumes that each cell has the same independent probability of containing an error. For example, if a spreadsheet has only 100 used cells (which is small), and assuming a moderate cell error rate of 3%, then the probability of at least one error in the spreadsheet is about 95%. This aligns well with experience.

The more used cells a spreadsheet has, the more likely it is that there are many errors. Even with a low cell error rate of 1%, a spreadsheet that has 1,000 used cells has a 97% probability of having at least 5 errors, and the expected number of errors is 10.

In general, errors seem to occur in a few percent of all cells, meaning that for large spreadsheets, the issue is how many errors there are, not whether an error exists.