BigCat Research

Forecast vs evaluative SROI: which first?

Short answer: A forecast SROI estimates the social value a programme will create if it achieves its intended outcomes; an evaluative SROI measures the value from outcomes that have already happened.¹ If your programme has not started, or you have no outcome data yet, start with a forecast and use it to set up data collection; the SROI Guide says this is often the better way in.¹ If the programme has run and you hold reliable outcome data, an evaluative SROI is the one that can tell funders what actually changed.¹

Disclosure: BigCat Research sells social impact analysis and SROI services, so we have a commercial interest in this topic. The method described here comes from the published sources listed at the end, not from us.

Two questions, two points in time

The SROI method itself is the same in both cases. Social Return on Investment is described in its standard guide as "a framework for measuring and accounting for" a broad concept of value, using money as a common unit so that a ratio of benefits to costs can be calculated.¹ A ratio of 3:1 means £1 invested delivers £3 of social value.¹ That guide, dated January 2012, updates the original 2009 Cabinet Office guide.¹ It is now hosted by the Institute for Social Value, the UK network of Social Value International, which is where socialvalueuk.org now redirects.²

What differs is the moment you ask the question. The Guide names two types:¹

• Evaluative: "conducted retrospectively and based on actual outcomes that have already taken place".
• Forecast: "predicts how much social value will be created if the activities meet their intended outcomes".

Think of a household budget. Planning next month's spending and checking last month's bank statement use the same categories and the same arithmetic. One is a plan you can still change. The other is a record you have to explain.

UK government guidance draws the same line with different words. The HM Treasury Green Book (2026 edition) says "appraisal is distinct from evaluation": appraisal assesses a proposal before it is approved and implemented, and evaluation assesses an intervention after it has been implemented.³ A forecast SROI sits on the appraisal side; an evaluative SROI sits on the evaluation side.

Side by side

Neither column is the "real" SROI. They answer different questions, and a well-run programme often needs both, one after the other.

When a forecast SROI is the better fit

The Guide calls forecast SROIs "especially useful in the planning stages of an activity". They show how an investment could maximise impact, and they help identify what should be measured once the project is running.¹

Three situations point clearly to a forecast:

1. The programme is new or not yet funded. You are choosing between designs, or writing a proposal. Strategic planning, market testing and scoping contracts are all listed as forecast uses, as is assessing which bidder is likely to create the most value.¹
2. This is your first SROI and outcome data are thin. The Guide says a first forecast will be much less time-consuming than an evaluative analysis unless you already have the right outcome data.¹
3. You want to fix your data before it is too late. A forecast is the natural moment to change how you collect information, so that an evaluative study is possible later.¹

A small example of the third point comes from the Guide itself. For a childcare service, it suggests recording outcomes when parents collect their children, which folds measurement into a routine that already happens.¹ A community kitchen could do the same at the serving counter. A youth sports club could do it at registration. None of this needs a research team on site every week; it needs someone to decide early what to ask.

A forecast is only as good as its assumptions. The Green Book tells government appraisers to adjust for optimism bias by raising cost and duration estimates and lowering benefit estimates.³ The SROI Guide has no optimism-bias uplift, but its "do not over-claim" principle and sensitivity testing serve a similar purpose.¹ Treat a forecast ratio as a planning tool, not as evidence that the value was created.

If you want to see how the adjustments move a ratio before you commission anything, our free SROI calculator builds an illustrative forecast from your own inputs. Its result is a forecast-style estimate, not an assured SROI.

When an evaluative SROI is the better fit

An evaluative SROI is the stronger choice when the activity has run long enough for outcomes to appear and someone needs to know what happened. A funder checking a grant or a board deciding whether to renew is the usual audience; monitoring and contract management are the evaluative uses the Guide names.¹

The numbers describe change that people actually experienced, not change you hoped for. That makes it the version that can be checked against evidence, and the Guide advises comparing changes in your own ratio over time and examining the reasons for them.¹

The price is time and money. An evaluative study "could take several months", the Guide warns, though the time drops sharply if the organisation already produces good outcome data or keeps social accounts.¹ When records are missing, filling the gap can be expensive, because it may involve surveying people who are no longer in contact with the organisation.¹

To show what a programme changed, you need some idea of what would have happened without it. The Magenta Book, HM Treasury's evaluation guidance, says this may require a comparison group or "baseline data before implementation so that a counterfactual can be estimated", and that it is "particularly important to plan early" what evidence to collect.⁴ A baseline you forgot to collect in year one cannot be recovered in year three. This is the strongest practical argument for doing the forecast first.

Why the Guide recommends forecasting first

The Guide says it is often preferable to start using SROI by forecasting what the social value may be, rather than evaluating what it was, because this ensures you have the right data collection systems in place for a full analysis later.¹ The Green Book makes the parallel point for government, telling practitioners to consider "from the outset how the proposal will be evaluated".³

The Guide's own worked example follows this path. Wheels-to-Meals, a fictional organisation that runs a luncheon club for older people, is first analysed as a forecast SROI run by internal staff, using information from past experience.¹ The Guide also announces a supplement, Wheels to Meals: one year on, setting out how the organisation developed its approach after completing an evaluation against that initial forecast.¹

So the realistic answer to "which first?" is usually "forecast, then evaluate against it". The comparison between the two is itself useful. If the evaluative ratio comes in well below the forecast, the gap tells you which assumptions were too generous, and the next forecast gets better.

What both types share

Some things do not change with timing, and buyers sometimes assume they do.

• The adjustments. Both types account for deadweight (how much would have happened anyway), attribution, displacement and drop-off.¹ Drop-off, for instance, is usually a fixed percentage deducted each year from outcomes that last more than one year.¹ For how these adjustments move a result, see how deadweight, attribution, displacement and drop-off affect the SROI result.
• The limits of the ratio. The Guide says it is "not appropriate to compare the social return ratios alone" across organisations, because each analysis rests on different stakeholders and judgements.¹ A systematic review of 40 public-health SROI studies found ratios from 1.1:1 to 65:1 and reached the same conclusion about comparing them.⁵ The same review found no consensus on how to account for the counterfactual or on the right time horizon.⁵
• Independent review. Social Value International offers Report Assurance, an independent review of an SROI or social impact report against the Principles of Social Value.⁶ Its page does not distinguish between forecast and evaluative reports.⁶

Whether SROI is the right method at all is a separate question. If it is still open for you, read when SROI is meaningful and which assumptions it should use before deciding on timing. If you are weighing SROI against government-style appraisal, see SROI vs cost-benefit analysis.

A quick way to decide

Ask three questions in order.

Has the activity produced outcomes yet? If not, forecast. There is nothing to evaluate.

Do you hold outcome data you would trust in front of a funder? If yes, an evaluative SROI is within reach and may take far less than several months.¹ If no, a forecast now plus better data collection is often cheaper than reconstructing the past.¹

Who will read the result, and what will they decide? A board choosing between two designs needs a forecast. A funder deciding on renewal needs evidence of what happened. If you are writing a brief for an external provider, our guide on commissioning a social impact assessment or SROI lists what to include.

BigCat's SROI measurement service delivers the ratio together with the assumption set, proxy sources and sensitivity analysis behind it, and social impact analysis reports the change findings and the evidence level behind them.

FAQ

What is the difference between a forecast SROI and an evaluative SROI? A forecast SROI predicts the social value a programme will create if it meets its intended outcomes. An evaluative SROI is carried out afterwards and is based on outcomes that have actually happened. The method is the same; the timing and the evidence behind the numbers differ.¹

Should a new programme start with a forecast SROI? Usually yes. The SROI Guide says it is often preferable to begin with a forecast because it puts the right data collection in place for a later evaluative analysis. A forecast also helps with design choices while the programme can still change.¹

How long does an evaluative SROI take? The SROI Guide says an evaluative SROI is more time-consuming than a forecast and could take several months. The time drops considerably if the organisation already collects good outcome data or keeps social accounts.¹

Can a forecast SROI ratio be reported as an achieved result? No. A forecast estimates value that would be created if outcomes are met, so it is a planning figure. UK government guidance also notes that appraisals made before implementation tend to be over-optimistic about key assumptions.¹³

Can I compare my SROI ratio with another organisation's ratio? Not on its own. The SROI Guide says comparing social return ratios alone is not appropriate, because each study involves different stakeholders and judgements. Compare your own ratio over time, and read any ratio together with its assumptions.¹

Author: BigCat Research team. Last reviewed: 15 October 2026.

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