0751 - h122 - basic health physics - 10 - counting statistics.objectives zto review the need for...
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Counting Statistics
Applied Health PhysicsApplied Health Physics
Oak Ridge Associated Universities
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Objectives
To review the need for statistics in health physics.
To review measures of central tendency and dispersion.
To review common statistical models (distributions).
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To review common statistical models (distributions).
To calculate the uncertainty for a count and count rate.
To review the statistics for a single count (rate) versus those for a series of counts of the same sample.
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Introduction
It is impossible to directly measure radiation in every situation at every point in space and time.
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It is impossible to know with complete certainty the level of radiation when it was not directly measured.
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Introduction
One of the goals of statistics is to make accurate inferences based on incomplete data.
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Using statistics, it is possible to extrapolate from a set of measurements to all measurements in a scientifically valid way.
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Introduction
Statistical tests allow us to answer questions such as:
How much radioactivity is present?
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y pHow accurate is the measured result?Is the detector working properly? (Are the detector results reproducible (precise))?Is there bias error in addition to random error in a measurement?Is this radioactive?
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Introduction
Without statistics, it is difficult to answer these questions.
St ti ti ll t ti ith
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Statistics allow us to answer questions with a degree of confidence that we are drawing the right conclusion.
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PART I:
DEFINE THE TERMS
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Uncertainty (Error)
Uncertainty (or error) in health physics is the difference between the true average rate of transformation (decay) and the measured rate.
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Accuracy refers to how close the measurement is to the true value.
Precision refers to how close (reproducible) a repeated set of measurements are.
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Uncertainty
Random errors fluctuate from one measurement to the next and yield results distributed about some mean value.
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Systematic (bias) errors tend to shift all measurements in a systematic way so the mean value is displaced.
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Uncertainty
An example of random error results from the measurement of radioactive decay.
Th f di ti d i d
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The process of radioactive decay is random in time.
An estimate of how many atoms will decay can be made but not the ones that will decay.
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Uncertainty
Some examples of bias error are:
Dose-rate dependence of instruments
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Throwing out high and/or low data points (outliers)
Uncertainty of a radioactivity standard
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Uncertainty
Uncertainty results from:
Measurement errors
S l ll ti
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Sample collection errors
Sample preparation errors
Analysis errors
Survey design errors
Modeling errors
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Assumptions
Nuclear transformations are random, independent, and occur at a constant rate.
The physical half life is long compared to the
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The physical half-life is long compared to the counting time.
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Science, vol. 23414
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Descriptive Statistics
Descriptive statistics characterize a set of data.
Descriptive statistics include:
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Descriptive statistics include:Measures of central tendency.Measures of dispersion.
Descriptive statistics should always be performed on a set of data.
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Measures Of Central Tendency
The mean is the arithmetic midpoint or more commonly, the average.
If the true mean is known it is designated as
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If the true mean is known, it is designated as μ.
For example, the true mean age of the people in this classroom could be determined.
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Central Tendency
However, the true mean of radioactive samples is not usually known.
A ti t f th i d f
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An estimate of the mean is made for radioactive samples.
The estimated mean is designated .X
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Central Tendency
The estimated mean :
XX i∑=
X
•Xi = an observed count (result).
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nX = count (result).
•n = the number of observations (counts) made.
μ⎯→⎯∞⎯→⎯ Xn then , as
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Central Tendency
μ⎯→⎯∞⎯→⎯ Xn then , as
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This is the basis for pooling data, or counting more than once, etc.
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Central Tendency
The median is the point at which half the measurements are greater and half are less.
Th di i b tt f t l
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The median is a better measure of central tendency for skewed distributions like dosimetry data or environmental data.
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Central Tendency
For example, assume we wanted to calculate the mean salary of everyone in the classroom.
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The mean would probably be a good measure of central tendency.
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Central Tendency
But suppose Bill Gates walked into the classroom.
The mean salary would not reflect my salary!
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The mean salary would not reflect my salary!
The median would be a better measure of central tendency for that severely skewed distribution.
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Measures of Dispersion
The range is the difference between the maximum and minimum values.
Th d i ti i th diff b t
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The deviation is the difference between a given value and the true mean, and can be positive or negative.
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Dispersion
The mean deviation is of little use since positive and negative values will cancel each other out.
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The variance is the square of the mean deviation, and assures positive values.
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Dispersion
The true variance is:
X i∑ −=
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)( μσ
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The true standard deviation is:n
σ
( )n
X i∑ −=
2μσ
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Dispersion
In health physics, the sample (group) variance and sample (group) standard deviation is used, since the true mean of radioactive samples is usually not known
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radioactive samples is usually not known.
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Dispersion
The sample (group) variance is:
( )1
2
2
−−
= ∑n
XXS i
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The sample (group) standard deviation is:
( )1
2
−−
= ∑n
XXS i
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Distributions
Distributions are statistical models for data.
They are used as theoretical means to l l t t ti ti lik th l ( )
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calculate statistics, like the sample (group) standard deviation, for example.
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Distributions
The binomial distribution is a fundamental frequency distribution governing random and dichotomous (2-sided) events.
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This fits radioactivity well, since transformation is random and dichotomous.
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Distributions
The Poisson distribution is a special case of the binomial distribution in which the probability of an event is small and the sample is large.
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The Poisson distribution also fits radioactivity very well, since the probability (p) of any one atom transforming is small, and a sample usually consists of a large number of atoms (>100).
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Distributions
The Poisson distribution is simple to use as a model, because it is characterized by one independent parameter, the mean.
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np=μ
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Distributions
The standard deviation is derived from the mean:
μσ =
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For example, μ is the count, and σ is the square root of the count.
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PART II:
SINGLE COUNT
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Counting Statistics for a Single Count
The Poisson distribution is important for counting radioactive samples.
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We assume that a single count is the mean of a Poisson distribution, and the square root of the count is the standard deviation.
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Single Count
For example, a single count of a sample was:
counts 209=X
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The standard deviation of that count is:
counts 5.14209 ==σ
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Single Count
This means that 68% of any repeated counts of this sample would be between:
t514209 ±X
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counts 5.14209 ±=X
5.2235.194 ≤≤ X
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Single Count Rate
Many times in health physics, counting results are expressed as a count rate (R) rather than a total count (X) by dividing by the count time (t)
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count time (t).
tXR =
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Single Count Rate
Continuing with the previous example, if the count time t were 10 minutes, the count rate (R) would be:
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cpmctsR 9.20min10
209 ==
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Single Count Rate
The standard deviation of the count rate (σR) can be calculated two ways:
XR =σ
Ror R =σ
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The next slide shows how these two equations are equivalent.
tRσ tR
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Single Count Rate
tRt
tXR
R ==
==
σσ then t
X :Xfor Rt Substitute
XRtthen
R
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( )2
2 :sidesboth Squaring ⎟⎟⎠
⎞⎜⎜⎝
⎛=
tRt
Rσ
( )tR
tRt
R ==2
2 :equation theReducing σ
tR
R =σ :sidesboth ofroot square theTaking
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Single Count Rate
tX
R =σtR
R =σ
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cpm
counts
R
R
4.1
min10
209
=
=
σ
σ
cpm
cpm
R
R
4.1
min10
9.20
=
=
σ
σ
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Single Count Rate
When calculating the standard deviation for a count rate, many people feel like they are making a mistake because they are “dividing by t twice”
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by t twice .
But this is correct.
ttX
tR
R
)(==σ
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Single Count Rate
So, for the previous example:
cpmcpmR 4.1
min10
9.20 ==σ
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The result would be expressed:
cpmcpm 4.19.20 ±
cpmRcpm 3.225.19or ≤≤
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Coefficient of Variation
Sometimes it is convenient to express the uncertainty as a percent of the count.
Thi i ll d th ffi i t f i ti
45
This is called the coefficient of variation (COV) or the percent variation (%error).
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Coefficient of Variation
For a given count:X
XX
σσ 100% =
( ) ( ) ( )XX
222 100100
%:sidesbothSquaring ==σ
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( )XXX 2
%:sidesboth Squaring σ
XX100
% :sidesboth ofroot square theTaking =σ
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Coefficient of Variation
For a given count rate:R
RR
σσ 100% =
tX
X 100%thenSince == σσ
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( )( ) ( ) ( )
XXX
tX
tX
R
2
2
2
2
2
2
2
2 100100100
% :sidesboth Squaring ===σ
XR100
% :sidesboth ofroot square theTaking =σ
tXt RR %then ,Since == σσ
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Coefficient of Variation
Continuing with the previous example:
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%9.6209
100% ==Rσ
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Coefficient of Variation
Note that the COV decreases with increasing values of the count.
F l if th t 809 i t d f
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For example, if the count was 809 instead of 209:
%5.3809
100% ==Rσ
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Coefficient of Variation
The COV can be reduced to an arbitrary value by increasing the counting time:
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2
%
1001⎟⎠⎞
⎜⎝⎛=
σRt
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Coefficient of Variation
For example, suppose a 1% uncertainty (error) was desired for the previous example:
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Some survey instruments now display %error, so you can count until you reach the desired % error.
hours 8almost or min4781
100
9.20
12
=⎟⎠⎞
⎜⎝⎛⎟⎟⎠
⎞⎜⎜⎝
⎛=
cpmt
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Uncertainty of Net Count Rate
Often in health physics, measurements are expressed as net count rates.
RRR
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There is uncertainty associated with both Rg and Rb.
bgnet RRR −=
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Net Count Rate
Uncertainties cannot simply be added or subtracted when two numbers are combined.
Th t l l t th bi d
53
The proper way to calculate the combined uncertainty with more than one term is to sum the squares of the uncertainties, then take the square root.
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Net Count Rate
For example, the uncertainty of a net count rate is:
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22
bg RRRnet σσσ +=
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Net Count Rate
RRRnet tR
bg
⎞⎛⎞⎛
=+= σσσσ
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R22 and
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b
b
g
gRnet
b
b
g
gRnet
tR
tR
tR
tR
+=
⎟⎟⎠
⎞⎜⎜⎝
⎛+⎟
⎟⎠
⎞⎜⎜⎝
⎛=
σ
σ2
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Net Count Rate
For example, suppose the gross count of a sample was 2,000 counts for a 10 minute count.
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The background was 200 counts for a 5 minute count.
What is the net count rate and associated uncertainty?
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Net Count Rate
cpmmcounts
mcountsRnet 160
5
200
10
000,2 =⎟⎠⎞
⎜⎝⎛−⎟
⎠⎞
⎜⎝⎛=
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mm 510 ⎠⎝⎠⎝
cpmmcpm
mcpm
net 3.5 5
40
10
200 =+=σ
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Net Count Rate
The result would be expressed:
cpmcpm 3.5160 ±
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pp
cpmRcpm net 3.165 7.154 ≤≤
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Net Count RateSpecial case: Rate-meters
The uncertainty associated with a reading on a count rate meter is:
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constant x time2
ratecount =crmσ
Time constant = response time
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Net Count Rate
The optimum distribution of a given total counting time between the gross count and background count can be determined by determining the ratio:
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determining the ratio:
b
g
b
g
RR
tt
=
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Net Count Rate
Using the previous example:
=RR
tt
b
g
b
g
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Then the time allotted for the gross count should be 2.23 times that allotted for the background.
23.2 40
200 ==cpmcpm
tt
b
g
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PART III:
MULTIPLE COUNTS
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Counting Statistics for Several Counts
Normally a radioactive sample would be counted only once, and a Poisson distribution assumed.
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In other words, the standard deviation is calculated from the mean.
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Several Counts
However, to determine if uncertainty is due only to the random nature of radioactivity and not other sources, the standard deviation can be determined experimentally from a series
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be determined experimentally from a series of counts.
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Several Counts
This is the approach taken in quality control testing of equipment.
Th di t ib ti f t f l
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The distribution of counts from a sample repeatedly counted is best described by a Gaussian (or normal) distribution.
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Several Counts
The Gaussian distribution is familiar to many people as the “bell-shaped” curve used in grading.
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The percentage of counts that will fall within a given range around the mean are:±1σ 68.3%±2σ 95.5%±3σ 99.7%
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Several Counts
The Gaussian distribution can be described by the:
MX
X i∑=
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Mean.
Standard deviation.
nX =
( )1
2
−−
= ∑n
XXS i
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Several Counts
For a sufficiently large sample (mean > 30), the Poisson distribution approximates the Gaussian distribution.
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Therefore, if the only uncertainty is from radioactivity, the standard deviation of a series of counts of the same sample determined assuming a Gaussian distribution should nearly equal the standard deviation determined assuming a Poisson distribution.
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Several Counts
If the two standard deviations are not nearly equal, then other factors (equipment problems) are contributing to uncertainty, and should be eliminated
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should be eliminated.
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Several Counts
There are several ways to determine “nearly equal”.
Th Chi S T t i th d
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The Chi-Square Test is one method.
The other method is the Reliability Factor, and we’ll use it in the statistics lab exercise.
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Several Counts
The Reliability Factor is defined as the standard deviation determined by Gaussian statistics divided by the Poisson standard deviation
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deviation.
X
SFR =..
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Several Counts
The RF versus the number of repetitions is then plotted on a graph such as the one in your Radiological Health Handbook.
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One of three conclusions can be drawn from the position of the RF value on the graph.
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Several Counts
If the point lies within the inner set of lines, the uncertainty is due only to the random nature of radioactivity.
If the point lies outside the outer set of lines
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If the point lies outside the outer set of lines, there is a strong indication that other uncertainty is involved (instrument calibration, etc.).
If the point lies between the lines, the result is inconclusive and needs to be repeated.
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Repetition Counts
1 209 449
2 217 174
3 248 317
4 235 23
5 224 38
( )2XXi −
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6 223 52
7 233 8
8 250 392
9 228 5
10 235 23
Total 2302 1481
Mean 230.2
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Several Counts
Gaussian: 2.23010
2302 ==X
8.129
1481 ==S
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Poisson:
9
2.152.230 === Xσ
2.23010
2302 ==X
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Several Counts
The RF is:
84.02.15
8.12 ==RF
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From the graph, this is an acceptable RF for 10 repetitive counts.
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References
Cember H. Introduction to Health Physics(1996).
Gollnick, DA. Basic Radiation Protection Technology, 4th edition (2000).
Knoll, GF. Radiation Detection and Measurement, 2nd edition (1989).
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National Council on Radiation Protection and Measurements. NCRP Report #112 Calibration of Survey Instruments Used in Radiation Protection for the Assessment of Ionizing Radiation Fields and Radioactive Surface Contamination (1991).
Shleien, BS; Slaback Jr., LA; Birky, BK. Handbook of Health Physics and Radiological Health, 3rd edition (1998).