# Core concepts

A handful of terms recur throughout the portal. This page defines them once; later
chapters assume them.

## Trial structure

Trial
: The whole study: its identity, its arms, its phases, the questionnaires, the audio,
  and the participants. A trial moves through a lifecycle from *Draft* to *Completed*
  (see {doc}`participants-lifecycle`).

Arm (treatment)
: A distinct condition a participant can be in. There are four kinds:
  - **Active**: the therapeutic sound under test.
  - **Sham**: a control sound, paired with the active arm it mirrors so blinding
    works.
  - **Washout (nothing)**: a rest period with no listening at all.
  - **Washout (unmodulated)**: a rest period where participants still listen, but to
    a plain, unmodulated sound.

Phase
: A stage of the trial over time, for example *Baseline*, an *Intervention* period,
  a *Washout*, and the post-trial *Debrief*. A trial is a sequence of phases; each
  intervention/washout phase is filled by an arm.

Randomisation sequence
: The ordering of arms a participant follows. In a **parallel** trial each participant
  is placed in one arm and stays there. In a **crossover** trial every participant
  passes through all the arms, in a randomised order, each order is a sequence.

## Recruitment

Completion target
: The number of **participants** you expect to *complete* a given arm. It is a soft
  goal, never a hard cap, an arm may overshoot it. Targets also set how likely a new
  participant is to be assigned to each arm. See {doc}`enrolment`.

Estimated completion rate
: The fraction of enrolled participants you expect to finish (default 0.5). It turns
  your completion targets into a total recruitment ceiling.

## Questionnaires

Assessment
: A questionnaire in the reusable library, its questions, how they are scored, and
  when they are asked. The same generic system renders every questionnaire in the app.

Timing
: *When* an assessment is asked, for example before treatment, at the start or end of
  a treatment period, every day, or at the end of the trial.

## Audio

Sound type
: The category of therapeutic sound (for example an amplitude-modulated tone).

Hearing profile
: A preset describing a participant's hearing (for example Mild, Moderate), used to
  pick the right sound file.

Modulation band
: The frequency band a sound is modulated in (for example 2–4 kHz). An *unmodulated*
  sound has no band.

These three together identify each uploaded sound; see {doc}`audio-pipeline`.