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Time Frame
The data presented are for two specific time periods—a
1-year period from January 1993 through December
1993, and a 7-year period from January 1987 through
December 1993. The reader will see that, with few
exceptions, the 1-year and 7-year data are remarkably
similar, with few changes in percentages.
Relationship of ASRS Data to All Aviation Incidents
ASRS reports are voluntarily submitted and are not
obtained through a statistically valid sampling process.
Thus, the ASRS cannot specify the relationship between
its database and the total volume of aviation
safety incidents that occur, nor can it say with certainty
that this relationship has remained fixed over time.
This is known as the self-reporting bias problem.
However, the ASRS can say with certainty that its database
provides definitive lower-bound estimates of the
frequencies at which various types of aviation safety
events actually occur. For example, 29,434 altitude overshoots
were reported to the ASRS from January 1987
through December 1992. It can be confidently concluded
that at least this number of overshoots occurred during the
1987-92 period—and probably many more. Often, such
lower-bound estimates are all that decision makers need
to determine that a problem exists and requires attention.
Known Biases
We are aware of two prominent factors that bias ASRS
statistical data. The first is the relatively high number
of reports received from pilots (currently about 96 percent
of ASRS report intake) versus controllers (roughly
3 percent). This imbalance causes the ASRS database to
have many more records describing pilot errors (altitude
deviations, runway transgressions, etc.) than controller
errors (operational errors, coordination failures, etc.).
The second biasing factor is the computerized error
detection capabilities at FAA Air Route Traffic Control
Centers (ARTCCs). These are very effective at capturing
altitude and track deviations that result in a loss of
aircraft separation. Thus, the ASRS receives disproportionately
large numbers of reports describing these
kinds of events, mostly from pilots.
Number of Reports vs. Number of Incidents
Many incidents are reported by more than one individual.
For example, an incident may be reported by a pilot and a
controller, several pilots and several controllers, the entire
flight crew of a given aircraft, and pilots of more than one
aircraft. In 1993, ASRS received 30,303 reports describing
24,348 unique incidents; thus, 5,955 reports
were “secondary,” in that they described incidents
which had already been reported to the ASRS.
Issue Number 6 27
Total and Percent Distributions
Multiple entries are permitted in many of the data fields
coded by ASRS analysts. For example, an altitude bust
that resulted in a loss of standard separation would be
coded in the Anomaly field as an altitude deviation, an
airborne conflict, and an ATC clearance violation. While
this is the most accurate way of coding events, it means
that incidents do not fall into neat, mutually exclusive
categories that always add up to 100 percent. Moreover,
it is not unusual for selected data fields to be left blank
during coding, either because needed information is not
available, or because the field is not deemed relevant to
a particular report. This presents an added complication
when incidents are totalled and percent distributions
are calculated.
The first chart in the following pages shows the number
of unique incidents reported to the ASRS over the
past 7 years. This provides a baseline for interpreting
data in succeeding charts which characterize the time,
location, and other aspects of the reported incidents.
The data in these latter tables are presented in a
consistent format that provides for unknown or inapplicable
data, and for cases in which more than one
category applies. An example is shown above in the
hypothetical table.
In this example, incident records are categorized as A,
B, or C. Any incident may be placed in one, two, or even
three of these categories. If categories A, B, and C are
simply added together, incidents that are recorded in
more than one category will be double-counted in the
“Total Row.” Since double-counting is usually unwanted
in summations, the totals have been adjusted to eliminate
double-counted events. The results are presented
in the row entitled Total Unique Incidents.
Thus, in the Hypothetical Example Table, a total of 165
incidents were reported during the current time period.
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