Impact Measurement Methodology

1. What we measure

Four figures carry TEEI's public claims: completed sessions, verified conversation hours, member countries, and active volunteer and learner countries. Each is defined below, and each is read from system records rather than reported by hand.

Two decisions determine what the headline numbers mean. Both were made deliberately and are recorded in the data contract rather than left implicit.

The stricter choice was taken in both cases. Conversation hours count only time both people were demonstrably present, which is a smaller number than scheduled time and a much smaller number than registrations.

2. Data sources

The figures come from two systems, and the boundary between them matters because they are merged, never joined.

Neither source is a registration count. A person signing up produces no session, no hour and no attendance record. Country figures are the exception: they are registration-based, as section 6 describes.

The two halves are added, not joined. The historical records live in quarantined tables the live query is structurally forbidden from reading, which is what makes double counting impossible rather than merely unlikely.

3. Definition of a completed session

A completed one-to-one booking on the earlier platform, or a booking on our own platform whose attendance record shows both people present past the floor. Group attendance is a separate measure.

Two conditions must both hold. The booking status must have reached completed, and the attendance settlement written afterwards must independently agree, with shared presence above the floor. A booking marked complete by a person, with no measured attendance behind it, is not a session here.

4. How attendance is verified

Attendance is derived from the video infrastructure's join and leave events, not from anyone marking a box. For each session the system takes both participants' connected intervals, sorts them, and computes the overlap: the time both people were in the room at the same time. Reconnections do not double count, and adjacent intervals that never overlap contribute nothing.

The floor is enforced in four independent places, so no code path can forget it: a database constraint that refuses to store a completed settlement below it, a single shared query predicate every reader must use rather than restate, an object-level check on the same rule, and the same threshold in the group-session path. A session below the floor is therefore not filtered out at reporting time; it never becomes a completed session in the first place.

5. How hours are calculated

One basis on both sides. On the earlier platform, the minutes both participants were in video on completed bookings. On our own platform, the shared connected time stored per session: the overlap of the two participants' connected intervals, credited up to the booked window plus 30 minutes' grace. Both measure the same thing, the time both people were in the room together. The earlier mixed basis, on which live sessions counted their scheduled duration, is retired.

Both eras therefore answer the same question: for how long were both people actually in the room together. Scheduled duration is kept as a separate, secondary figure and is never published as conversation time.

One consequence worth stating plainly: because time is floored and capped, and because sessions below the floor contribute zero, the published hours figure is a lower bound. It understates rather than overstates.

6. How participants and countries are counted

A member is a profile on the earlier platform with a valid country code, or a volunteer or learner record on our own platform whose country can be resolved.

An active volunteer appeared as the adviser in at least one booking, or published an offering. On the earlier platform that follows the activity recorded there; on our own platform it follows the profile. The two are not the same test, so this figure is never described as all recently active volunteers.

Country figures are counts of countries, not of people, and the basis is residence: where a person is, not citizenship or affiliation. A Ukrainian in exile is recorded where she actually is. The basis has not changed since the first version of the contract.

127 plus 65 is more than 148 because the two sets overlap. 64 countries have both, so their union is 128. Both are subsets of the member set. The remaining 20 countries have registered members but no recorded session on either side, because member is a registration with a valid country while active requires having appeared in a booking.

7. Duplicate and invalid-record handling

Bookings that did not result in a verified session are excluded, not adjusted. For the historical language dataset the excluded statuses were:

On deduplication:

On the earlier platform, member counts are per profile per group: a person in both the language group and the mentorship group counts once in each. Matching one person across the two platforms is best effort and follows the organisational record.

On the live platform each booking can carry at most one attendance settlement, enforced by a primary key, so a session cannot be counted twice. Database triggers additionally forbid a settlement whose outcome disagrees with its booking's status, and forbid rewriting a booking's status once a final settlement exists.

Also counted: staff, internal and test accounts are not filtered out of the live query, so a real session between a TEEI staff member and a participant counts like any other. A person active on both platforms is counted once in each era.

8. Programme coverage

The headline figures cover one-to-one sessions across TEEI Language and TEEI Mentorship, counted as a single number. Group sessions are measured but reported separately and are never added to the totals.

A group session can be marked ended automatically without anyone having attended, so its scheduled duration says nothing about attendance. At the snapshot, ended clubs carried 12.78 scheduled hours and no participant had passed the five-minute floor; four had no enrolments at all. Those hours stay out of the totals.

9. Known data limitations

Each of these is recorded in the contract's exception ledger, with its effect on the figures.

A limitation we corrected

An earlier version of this dataset reported roughly 500 bookings lost to pagination drift. That was wrong, and the correction is part of the record: it was a parameter bug in our own extraction code, not data loss. The complete pull is 16,312 of 16,312 language bookings, no page missed and 863 of 863 mentorship bookings, no page missed. A second early claim, that the measured-time field was unset on hundreds of sessions, was also wrong: the field was never unset, and the sessions in question carry a measured zero.

10. Validation and integrity checks

The dataset is rebuilt by a script that asserts 17 invariants and aborts rather than emit numbers that fail any of them. Two consecutive builds have produced identical checksums. Every source file is recorded with its size and SHA-256.

Separately, the published country counts are frozen by assertion I13. If a rebuild produced different numbers, it would fail rather than quietly publish them.

11. Reporting period and update frequency

This is why the live totals below differ from the snapshot figures in the contract, and why both are shown. The method has not changed; the sessions have accumulated.

12. Dataset version and citation

How to cite

The Educational Equality Institute (2026). TEEI Impact Measurement Methodology, dataset version 2.3.0. https://theeducationalequalityinstitute.org/about/impact/methodology/

@misc{teei_impact_methodology,
  author       = {{The Educational Equality Institute}},
  title        = {TEEI Impact Measurement Methodology},
  year         = {2026},
  version      = {2.3.0},
  howpublished = {\url{https://theeducationalequalityinstitute.org/about/impact/methodology/}},
  note         = {Historical half frozen at 2026-07-23; live figures at /api/impact-stats.json}
}

13. Relationship to our other published figures

TEEI publishes other numbers elsewhere, and they are not interchangeable with these. The relationships are fixed and are stated here so the figures are not accidentally combined.

Conversation hours are part of volunteer hours: they measure time in video between two people, while volunteer hours also include preparation, coordination and everything else volunteers give.