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without missing anything. It can be very time-consuming and/or laborious if many
behaviours or subjects are involved, and the tempo is rapid. However, a sheet can be
designed to simplify data collection.
3.8.3 Instantaneous and Scan Sampling
These are time-sampling based systems in which the observer records the behaviour
state at the instant ending a predefined interval – for example, on the minute – which
is usually signalled by an auditory device heard only by the observer. By using such
a device, the observer is not having to constantly check the time, and can concentrate
on observing. To avoid bias, the observer only records what is going on at that point.
A potential problem is the difficulty in identifying the behaviour at a single glance.
Instead, it may require observing for 5 seconds to gauge what is occurring. The
observer can then record the behaviour at the end of the 5 seconds.
Instantaneous sampling is used where one subject is observed, while scan sampling
involves a group of subjects. In scan sampling, the observer must scan the group to
record the behaviour of all individuals. To avoid bias, it is important that even the
scan is systematic (for example, from left to right).
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Instantaneous and scan sampling approaches are the easiest ways of estimating the
percentage of time spent in specific activities. Thus, it is well suited to studies of
workload. It is less useful for data on interactions since they often occur in sequences
that cannot be recorded in a single scan. In addition, this method also misses
infrequent behaviours of short duration, unless the interval between samples is very
short or the entire observation is long. It is, however, a relatively simple method, and
naive observers can be quickly trained in its use.
The appropriate interval length depends on several factors, including activity level
(how often behaviour changes), group size (longer for more), or whether a single or
mixed sampling strategy is adopted (most common in trained observers). The shorter
the interval, the more the data represents that which would emerge with continuous
sampling. Shorter intervals also mean more data to analyse, but it is important in
rapid tempo operations. Longer scan intervals can be used during low tempo
situations. However, they need to be combined with some continuous sampling to
ensure that brief but important behaviours are recorded.
3.8.4 One-Zero Sampling
In one-zero sampling, time intervals are also used. However, each behaviour during
the interval is recorded as having an arbitrary score of 1, regardless of its actual
frequency. For example, a frequency of both 5 and 3 are recorded as “1”, while 0 is
entered if the behaviour was not observed. This does not allow the calculation of true
duration, rate or percentage, so it is not advisable for data collection where high
fidelity is important. It also over-estimates true percentage, when the results are
compared with continuous data. However, it is easy, and does produce high interobserver
reliability. Consequently, it may be the appropriate method to adopt when
trained observers are not available, or for getting estimates of behaviour levels.
3.9 Choosing the Sampling Period and Focusing the Observations
It is useful to divide observation periods into equal length sample periods. This
assists check-sheet design, and data analysis. A basic observation period includes at
least one complete replication of data collection: that is, each subject is observed a
minimum of once in random order. In general, an observation period will last for ½ -
1 hour during faster tempo operations, and 2-3 hours when things are occurring at a
slower tempo. As a rule of thumb, the duration should be shorter than the fatigue
threshold of the observer. This is no longer than 2 hours in high tempo situations,
and increases as the tempo decreases.
In addition, in a field/operational setting, the observer cannot pay attention to
everything at the same time. Thus, during a sampling period, the observer restricts
their focus. The most common focus is on a single individual (focal subject) such that
all behaviours initiated by that subject are recorded. This may also include
interactions that the focal subject has with other individuals. The focus, which can be
any of the following, needs to be predetermined in the methodology:
1. Focal subject: Individual selected from total group, for example, Duty Officer
(most effective if used with continuous sampling).
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2. Focal subgroup: Related sub-set selected from total group, for example, the Duty
Clerk and Duty Officer within the Operations Cell (most effective if used with
　
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