What is reference class forecasting?
It is a method of forecasting a project's cost or duration from the actual outcomes of a group of similar past projects, rather than from the details of the current plan. Kahneman and Lovallo called this the outside view. Flyvbjerg applied it to infrastructure planning. For a small fit-out, the practical version is to ask what happened on comparable jobs, not what the programme says should happen.
From the RRD Field Guide, checked against the sources cited in Cheapest Quote vs Delay Risk: The Economics of Choosing a Contractor in Singapore. General information, not advice for your project.
The fuller picture
The cheapest quote is only cheaper if the job finishes on time and on scope. Overrun research shows delays are common and fat-tailed, so a small price saving can be wiped out by a few weeks of rent, lost revenue or holding-over. Compare quotes on expected total cost, including probability-weighted delay cost and the worst plausible outcome, not the tender price alone.