A scenario is one merger put to one set of sites: who is buying whom, how far customers travel, and how shares are counted. You set it up in the pane on the left of the Scenarios page, run it, and Sloc builds a local market around every party site and works out the shares in each one.

Step 1 - Choose the subset to analyse

Subset to analyse sets which sites the scenario covers, and the data version those sites come from. Underneath, Sloc shows how many sites and fascias are in it. If a fascia you expect is missing later on, it isn't in the subset.

Step 2 - Name the merging parties

Choose the Acquirer, then one or more Target(s). Each fascia shows how many sites it has in the subset, so you can see the size of the transaction before you run anything.

By default every site the target owns is in play. Switch to Specific sites if only part of the estate is being sold - a window opens with the target's sites, which you can filter and search, and you tick the ones the deal covers.

Step 3 - Decide where the markets sit

A local market is a catchment drawn around one site. Centre local markets on decides which sites get one, and the option tells you how many markets that makes:

• All party sites - a market around every acquirer and target site. The fuller picture, and the usual starting point.

• Target sites only - a market around the target's sites alone. Fewer markets, quicker to run, and often all you need for a first look.

Analyse no-overlap markets in full also works out shares for markets where only one party is present. It's slower, and with a routed catchment it uses more of your routing allowance, so leave it off unless you need those figures.

Step 4 - Define the catchment areas

Type sets how the catchment is drawn. Straight-line distance is a simple radius - instant, and free of routing. Street-wise distance and Travel-time duration follow the road network, which is more realistic but takes longer and draws on your routing allowance.

Size is the radius, distance or time itself. Symmetric gives every market the same catchment; Asymmetric lets you vary it - a large out-of-town store pulling further than a convenience shop, say. The limits for each type are in how to define catchment area in Sloc Local.

Step 5 - Choose how shares are counted

Under Configure market concentration, Share of sites counts each site as one - fine when the sites are broadly alike. Weighted share lets you give sites different weight, by floorspace, format or distance from the centre of the market. See how to weight sites in a local scenario.

Step 6 - Run it

Click Analyse local markets. The button says how many markets it will analyse. Results appear on the right, split into Overlap - markets where both parties are present - and No overlap, where only one is.

Pro tips

• Start with a straight-line catchment. It costs nothing and runs instantly, so you can settle the parties, the sizes and the metric before you spend routing on the version you'll rely on.

• Centring on target sites only is the quickest way to see whether a deal has local problems at all. Widen to all party sites once you know it does.

• Save the scenario before you change it, so each variant can be compared later rather than rebuilt - see how to save, open and share scenarios.

• Once it's run, how to read your All local markets results covers the map, the table and the risk filters.