Notes for owners · Software Audit and Digital Transformation
What a Parallel Run Involves Before You Switch Off Spreadsheets
A parallel run means entering the same work in the old sheets and the new system for an agreed period, then comparing what each produces on the same days. It doubles the entry effort. It also catches a misread rule while a working fallback still exists. GullySystem’s migration service plans it before cutover.
Ganesh HS, Strategy and Technology, GullySystem · · 3 min read
What a parallel run is for
Test loads show whether the data arrived. They cannot show whether daily work produces the same answers. A parallel run tests that.
Staff keep the old sheets going while also entering the same orders, receipts and stock movements in the new system. At set points, both are compared.
Differences point to something specific. A rate slab was read wrongly, a discount rule was missed or a unit was mapped to the wrong field.
What to compare, and on which days
Pick figures the business already trusts and checks anyway. Comparing them daily for the first stretch keeps each difference small enough to trace.
Weekly comparison may follow once daily figures agree. Keep a written log of every difference, its cause and how it was settled.
- Sales raised that day, by customer
- Receipts and outstanding balance per party
- Stock on hand for the items that move most
- Tax totals on the invoices issued
- Commission or incentive worked out by the sheet’s formula
Who carries the double entry
Entering everything twice is tiring. Usually two or three people do it, plus the accountant for balances. Plan their other work around it.
Choose a period without a peak season or a filing deadline. Tell the team in advance how long it lasts and what ends it. People keep double entry going better when they can see the finish.
How to decide the old sheets can stop
Agree the exit test before the run starts. A common test is that both produce the same figures on the same days across one full month-end.
Any remaining difference should be zero or explained in writing. The accountant signs off balances. The person who uses each report confirms it matches.
Then plan the switch. Freeze the sheets, load changes made during the freeze and write down the way back if the first days go badly.
- Exit test written and agreed in advance
- Every difference logged with its cause
- Accountant sign-off on party and stock balances
- Freeze window, final load and fallback route
When a parallel run can be shorter or skipped
A small, simple move may not need one. A contact list or a single reference sheet with no money in it can switch directly.
For anything that touches billing, balances or stock, keep the run. It is the step that turns cutover into a planned decision instead of a hope.
Parallel run comparison log
A daily sheet with the figure compared, the value from each system and the difference. Add columns for the cause, who settled it and the date. When the difference column stays empty across a month-end, the exit test is close.
Open a blank worksheet to printQuestions owners ask
Can we skip the parallel run to save time?
For a small move with no money or stock in it, often yes. Where billing, balances or stock are involved, skipping it removes the one check that compares real daily work.
What if the two systems keep disagreeing?
Trace each difference to a cause before moving on. Most come from a misread rule or a wrong mapping. Fix the rule, reload if needed and compare again on the next day.
Does a clean parallel run mean nothing can go wrong after cutover?
No. It covers the work that happened during the run. Rare cases, such as a year-end entry, may appear later, so keep support and the original files available after the switch.
When is a good day to cut over?
Usually just after a month-end or year-end close, at the start of a quiet week. The books are settled and less work is in progress.