What is a tabletop exercise, and what is an MSEL?
A tabletop exercise (TTX) is a discussion-based rehearsal of an emergency. The people who would respond to a real incident sit around a table, a facilitator describes a situation as it unfolds, and the group talks through what they would do, using their organization’s actual plans. Nothing is deployed and no equipment is touched. The point is to find gaps in plans, roles and communication while the stakes are low.
In the United States, many exercise programs follow HSEEP, the Homeland Security Exercise and Evaluation Program. It describes how to design, run and evaluate exercises, from small discussions to full-scale field drills. A tabletop is the most common discussion-based type.
What happens during one
A typical tabletop runs two to four hours:
- Briefing. Players get the ground rules, their roles and the starting situation.
- STARTEX. The exercise starts. The facilitator reads the opening situation.
- Phases. The scenario moves forward in phases, often with time jumps (“It is now 14:00, two hours later”). In each phase, players receive new information and have to decide what to do.
- ENDEX. The exercise ends.
- Hot wash. An immediate debrief: what went well, what did not, and what should change. Its notes feed the after-action report and improvement plan.
The MSEL
The Master Scenario Events List (MSEL, often said “measle”) is the exercise’s script and timeline. It is usually a table with one row per event:
| Time | From → To | Event | Expected action |
|---|---|---|---|
| 0:00 | Facilitator → All | STARTEX: normal morning operations | Review the incident response plan |
| 0:15 | Field staff (SimCell) → Operations | Phone call: a display shows the wrong data | Log a ticket, check the source system |
| 0:30 | Public (SimCell) → PIO | Social post: a rumor about the outage spreads | Decide whether and how to respond |
Each row says when something happens, who sends it, who receives it, what it is, and what the designers expect the players to do. Facilitators use the expected actions to evaluate the response and to steer the discussion.
Injects
An inject is one piece of information delivered to the players: a phone call, an email, a helpdesk ticket, a social media post, a radio message, a log file, a news report. Injects carry the scenario forward. Good ones look like what responders would really receive, so players react to the content instead of to the exercise.
Injects play different parts. Some move the core story along. Some are noise, the ordinary background traffic of a busy day. Some are red herrings that point the wrong way, some add pressure (a reporter calling, a manager asking for an update), and some force a decision.
Other terms you will hear
- SimCell (simulation cell): the facilitators who play everyone outside the room, such as the public, vendors, reporters or another agency.
- Controller handbook: the facilitator’s guide with the MSEL, scripts for SimCell calls, the rubric and the hot wash questions.
- Player briefing: what players receive before the start: the situation, their roles, and forms such as the ICS-201 (incident briefing) and ICS-203 (organization assignment list) from the Incident Command System.
- PIO: Public Information Officer, the person responsible for public statements.
- Objectives: what the exercise is meant to test, written before the scenario. Every inject should serve at least one.
Why they are hard to build
Writing a good tabletop takes days. Someone has to read the organization’s plans, invent a plausible incident that tests specific sections of them, write a timed MSEL, produce fifteen to twenty-five injects in a dozen formats, and prepare packets for facilitators and players. The injects that take longest are the ones that matter most in the room: a believable social media post, a ticket printout, a voicemail.
Action Stations drafts all of that from your own documents, with citations back to the plan sections each part tests, and leaves every word editable.