# Method: Elevated Plus Maze Anxiety Index Calculator (method version 1.0.0)

## Calculation

For each animal, from the session time, the time in the open arms, closed arms and center, and the number of entries into the open and closed arms:

1. **% open-arm time** = open-arm time / session time × 100.
2. **% open-arm entries** = open-arm entries / (open + closed entries) × 100.
3. **Anxiety Index** = 1 − [(open-arm time / session time) + (open-arm entries / total arm entries)] / 2, from 0 to 1; higher values mean more avoidance of the open arms (formula as printed in Hoffman et al. 2015).
4. **Total entries** = open + closed entries, a measure of activity.
5. **% center time** = center time / session time × 100.
6. **Time check.** The animal is flagged when open + closed + center time differs from the session time by more than 10% of the session time.
7. **Group statistics** per group: mean, standard deviation (n − 1 denominator) and SEM = SD / √n of each measure, over the animals whose value is computable.

## Conventions

- Times are in seconds; the session time defaults to 300 s when left blank.
- A measure whose denominator is 0 is not computable: with no arm entries, % open-arm entries and the Anxiety Index; with no session time, the time percentages and the Anxiety Index. The table shows "—", the CSV leaves it blank, and the animal is left out of that measure's group statistics.
- The 10% time tolerance is this tool's convention for catching scoring errors.

## Worked examples

Three cases in `cases.json`: the Anxiety Index worked by hand, a session whose zone times miss the session time, and an animal with no arm entries (edge case).

## References

- Hoffman JR, Ostfeld I, Stout JR, Harris RC, Kaplan Z, Cohen H. β-Alanine supplemented diets enhance behavioral resilience to stress exposure in an animal model of PTSD. Amino Acids. 2015;47(6):1247-1257. https://doi.org/10.1007/s00726-015-1952-y
- Walf AA, Frye CA. The use of the elevated plus maze as an assay of anxiety-related behavior in rodents. Nature Protocols. 2007;2(2):322-328. https://doi.org/10.1038/nprot.2007.44
- Pellow S, Chopin P, File SE, Briley M. Validation of open:closed arm entries in an elevated plus-maze as a measure of anxiety in the rat. Journal of Neuroscience Methods. 1985;14(3):149-167. https://doi.org/10.1016/0165-0270(85)90031-7

## Changes from the page before this record

- The page titled its result "Anxiety Index" but computed 1 − open-arm time / session time, which ignores entries. It now computes the published index, which averages the open-arm time share and the open-arm entry share. For the worked example the page gave 0.800; the published index is 0.775.
- With no arm entries, % open-arm entries was 0 and entered the group mean; with no session time the time measures were 0 and the index was 1. These values are now not computable and are left out.
- % open-arm time, % open-arm entries, total entries, % center time and the time check are otherwise unchanged.