# Method: NOR Discrimination Index Calculator (method version 1.0.0)

## Calculation

For each animal in the test phase, with N the seconds spent exploring the novel object and F the seconds spent exploring the familiar object:

1. **Discrimination index** DI = (N − F) / (N + F), from −1 (only the familiar object) to +1 (only the novel object); 0 means no preference (Ennaceur & Delacour 1988, index d2).
2. **Preference index** PI = N / (N + F), the share of exploration spent on the novel object; 0.5 means no preference.
3. **Group statistics** per group: mean, standard deviation (n − 1 denominator) and SEM = SD / √n, over the animals whose index is computable.

## Conventions

- Times are in seconds. In keyboard mode the tool adds time while the scorer holds F (familiar) or N (novel); in calculator mode the scorer enters the totals.
- An animal with N + F = 0 has no index. The table shows "—", the CSV leaves DI and PI blank, and the animal is left out of its group's statistics. A group with no computable animal has no mean.
- The tool applies no minimum-exploration criterion; laboratories that exclude animals below a threshold (often 20 s in total) apply it themselves and report it.

## Worked examples

Three cases in `cases.json`: a preference for the novel object, a preference for the familiar object, and an animal that explored neither object (edge case).

## References

- Ennaceur A, Delacour J. A new one-trial test for neurobiological studies of memory in rats. 1: Behavioral data. Behavioural Brain Research. 1988;31(1):47-59. https://doi.org/10.1016/0166-4328(88)90157-x

## Changes from the page before this record

- An animal that explored neither object got DI = 0 and PI = 0. That reads as "no preference" and entered the group mean. It now has no index and is left out of group statistics, and the CSV leaves its DI and PI blank.
- Results for animals that explored at least one object are unchanged.