Aerial delivery system—A fixed- or rotarywinged aircraft capable of delivering firefighting chemicals.

Aerial drop—A release of firefighting chemical from an aerial delivery system in flight.

Algorithm—A rule for solving a certain type of problem.

Analysis of variance—A statistical technique by which the total variation in a set of data may be reduced to components associated with the possible sources of variation, allowing the relative importance of each source to be assessed.

Contour plot—A graphical picture on which the characteristics of a surface are shown by contour lines. In drop testing, the isopleths join points of equal coverage level on a surface.

Correlation coefficient—A number between –1 and 1 that measures the degree to which two variables are linearly related.

Coverage level—A recommended amount in gallons of retardant applied to a specific area (100 square feet) of surface. Coverage level 2 represents 2 gallons per 100 square feet (gpc).

Crossrange—Perpendicular to the direction of flight.

Cross validation—A method of comparing predicted and observed values.

Data array—Data arranged in a matrix with columns and rows.

Distribution (frequency)—A frequency distribution shows the number of observations falling into each of several ranges of values. Frequency distributions are sometimes displayed as histograms.

Downrange—Parallel to the direction of flight.

Error (residual)—The difference between the predicted value and the observed value.

Firefighting chemicals—Chemical products such as long-term retardants and water enhancers (chemicals containing ingredients designed to alter the physical behavior of water) used in firefighting.

Fire retardant—Any substance, except plain water, that reduces the flammability of fuels or slows their rate of combustion.

Fire suppressant—An agent that extinguishes the flaming and glowing phases of combustion when applied directly to the burning fuel.

First quartile—The 25th percentile. After a set of values has been arranged in order of magnitude, the first quartile is the value that has 75 percent of the values below it.

Flow rate—The rate at which retardant exits a tank or bucket, usually expressed in gallons per second.

GPC—A unit for measuring coverage expressed in gallons per 100 square feet.

Grid—A physical array incorporating containers set in a regular, defined pattern to measure deposition patterns created by the aerial release of fire chemicals.

Ground pattern—The characteristics of ground deposition from aerially delivered liquid.

Histogram—A graph of a frequency distribution table in which rectangles with bases on the horizontal axis are given widths equal to the class intervals. The heights of the rectangles are equal to the corresponding frequencies.

Isopleth—A line drawn on a map through all points having the same numerical value.

Line length—The length, usually expressed in feet, of a ground pattern. Line length is used to relate the length of different coverage levels within a ground pattern.

Linear interpolation—Estimation of a value of a variable between two known values when it is assumed there is uniform change between the two known values.

Mean absolute error (MAE)—The average of the absolute value of a set of residuals.

Mean square error (MSE)—The average of a set of residuals after each one has been squared.

Median—The 50th percentile. After a set of values has been arranged in order of magnitude, the median is the value that has 50 percent of the values below it.

P-value—In a hypothesis test, the probability of observing an outcome “more contradictory to the null hypothesis than the observed sample result” is called the p-value (Ott 1993).

QQ plots—Quantile-quantile plots. A graph comparing the distributions of two variables.

Replicates—Duplicates. A replicate drop or a duplicate drop is one that has the same factor levels, specifically, the same height, speed, volume, flow rate, and so forth.

Residual (error)—The difference between the predicted value and the observed value.

Sampling—The process of selecting a sample for testing.

Sampling density—The number of samples in a fixed area.

Tare—The weight of the empty container.

Third quartile—The 75th percentile. After a set of values has been arranged in order of magnitude, the third quartile is the value that has 25 percent of the values below it.

Triangulation—A weighted linear combination used for estimating values at specific locations. The weights depend on the distance and location.

Variability—Data variability refers to the spread of values along the scale of measurement and the extent to which the data are grouped.