Scanning speed and timing

Also called Scan rate, Scan speed, First-item delay, Acceptance time, Post-selection delay

Scan rate, first-item delay, acceptance time and loops: the four settings that decide whether scanning is usable or exhausting.

What it is

Scanning has a handful of timing settings, and the difference between a person who scans fluently and a person who has "given up on switches" is very often in them. Scan rate is how long the highlight rests on each item. First-item delay is the extra pause before the first item after a selection. Acceptance time is how long the switch must be held before a press counts. Post-selection delay is the dead time after a choice when presses are ignored. Loops is how many times the scan cycles before it stops.

They are set once, at assessment, and then left for years while the person changes. That is the problem this entry exists to fix.

Who it can help

Every scanning user, and especially anyone whose accuracy has drifted: a child who has got faster and is now bored into missing, an adult with motor neurone disease whose press has slowed, a person on a new medication, a person who is fine at 10am and useless by 3pm.

It is a settings problem, so it will not help a person whose switch site is wrong or whose switch has moved. Rule those out first; they are more common than a timing fault.

How it works

Scan rate: a starting point is 1.5 to 2 seconds per item, faster for practised users (some run under half a second), slower for beginners or when tired. Too slow is as bad as too fast: attention drifts and the person presses on the item after the one they wanted.

First-item delay: after a selection the person needs to look back to the top and reset; an extra half to one second on the first item stops the "always miss the first row" pattern. Acceptance time: 0.1 to 0.3 seconds filters out a tremor or a bump without making the press feel unresponsive. Post-selection delay: half a second or so stops a bounce selecting twice. Loops: two or three, then stop and wait, so a person who missed is not chased round the page for ever. Some devices also offer a separate, slower rate for rows than for cells.

Getting started

This week, time it. Watch ten selections and count how many were the item after the one the person wanted (rate too slow, or first-item delay too short) versus the item before (rate too fast). Change one setting by a small step, watch ten more, and write down both numbers.

Then set two profiles if the device allows it: a fresh one and a tired one, and make switching between them part of the afternoon routine. And put a review of the timings in the diary every term or every three months, with the numbers to compare against.

Where to get help

Any speech and language therapist working in AAC can review timings; the specialised AAC services do it as part of ongoing support for their users. The ACE Centre and CALL Scotland publish set-up guides that name every one of these settings, and the manuals from Smartbox and Tobii Dynavox explain where they live in their software.