Types of Variables - Astrostatistics



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Random Variables, Probability Distributions, and Expected Values

Random Variables (RV’s): Numerical value assigned to the outcomes of an experiment. Capital letters X, Y, Z, with or without subscripts, are used to denote RV’s

Examples

• B – V colors of stars

• Absolute magnitude of quasars M_i in the i band

• Number of electrons emitted from a cathode in a time interval of length t.

Two types: Discrete and Continuous

a. Probability distribution of a discrete random variable: table of values of the variable and the proportion of times (or probability) it occurs (which may be expressible in functional form). The first two RV’s above are ‘continuous’.

b. Probability distribution of a continuous random variable: idealized curve (perhaps from a histogram) which represents probability that a value of the variable occurs as an area under the curve.

Example: Discrete Random Variable.

Consider observing some phenomena with exactly two possible outcomes (say, success and failure) until the first success occurs, when the phenomena are independent of one another. The it can be shown that the probability function of the number Y of trials until the first success occurs is given by

p(y|() = ( (1 - ()y-1. y = 1, 2, … and 0 otherwise. (geometric distribution)

The parameter ( is the probability of success. For example suppose we are looking for some astronomical object at random and count the number of objects examined until the first occurrence of the object is found

Expected Value of a Discrete RV.

The mean µ of a probability distribution or the mean of a random variable or the expected value of X is defined to be

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µ = E(X) = [pic] and more generally for a function g(x)

E[g(x)] = Σg(x) p(x)

In particular, the expected value of the RV X2 is given by

E(X2) = [pic]

The Variance [pic]of a RV X is given by (2 = Var(X) = E(X2) -[E(X)]2 ; and the standard deviation ( of X =SD(X) is defined to be ( = [pic].

A special discrete probability distribution we will encounter is the Poisson.

The 'Poisson Distribution'.

Situations in which there are many opportunities for some phenomena to occur but the chance that the phenomenon will occur in any given time interval, region of space or whatever is very small, lead to the distribution of the number X of occurrences of the phenomena having a Poisson distribution. The Poisson distribution has a parameter ( measuring the rate at which the phenomena occur per unit (time period, interval, area, etc.). Here are some examples:

1. Number X of earthquakes in a region (for example, California, Indonesia, Iran, Turkey, Mexico) in a specified period (five years?)of magnitudes greater than 5.0

2. Number X of times lightning strikes in a 30 minute period in a region (like the state of Colorado)

. 3. The arrival times of photons from a non-variable astronomical object.

4. The spatial distribution of instrumental background photons in an

image.

5.. The number of photons arriving in adjacent bins in a spectrum of a

faint continuum source.

6. The number of ‘arguments’ married couples have in one year.

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The probability distribution (frequency function) p(y) of a Poisson random variable with rate parameter ( is given by

p( y | ( ) = e-( (y/ y! , y = 0, 1, 2, . . . ,

Fact: The sum of independent Poisson random variables has a Poisson distribution with parameter the sum of the parameters of the individual variables: Assume Yi,

i = 1, 2, …, n, have a Poisson distribution with parameter (i. Then

Y = ( Yi has a Poisson distribution with parameter ( = ( (i .

The mean and variance of the Poisson distribution are both equal to (. For values of ( ‘large’, say ( > 25 (or even smaller), the Poisson distribution is approximately normal. A probability histogram of the Poisson distribution with ( = 25 is given below.

What does the distribution look like? Yeah, normal! So, if ( is large, one can approximate Poisson probabilities using the normal distribution with mean ( and standard deviation ((.

If a response variable in a regression context has a Poisson distribution, one can perform a ‘Poisson regression’ analogously to what one does if Y has a normal distribution in conventional linear or multiple regression. We will illustrate this later, as an example of ‘generalized linear models’.

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Continuous Random Variables

Definition. A continuous random variable X is one for which the outcome can be any number in an interval or collection of intervals.

Examples. Height, weight, time, head circumference, rainfall amounts, lifetime of light bulbs, physical measurements, etc.

Probabilities are obtained as areas under a curve, called the probability density function f(x). Below is a graph of the pdf f(x|20) = [pic], for x > 0 and 0 elsewhere--it is called the exponential pdf with mean µ =20; the standard deviation

is also 20. It could represent the lifetimes of batteries until re-charging,e.g..The cumulative distribution function CDF gives the total area under the curve (or cumulative probability):

CDF = F(x) = [pic]

= 1 – e-x/20, for x > 0 and 0 elsewhere

Areas under the curve between two points give the proportion of a population that have values between the two points. For example, Prob(10 < X < 30) = [pic]

= e-10/20 - e-30/20 = e-0.5 - e-1.5 . .

The Normal Distribution. The most well-known continuous distribution is probably the normal, with probability density function (pdf) f(x) given by

f(x|µ, σ) = [pic], -∞ < x < ∞ and CDF Φ(x) = [pic]

The graph of a normal pdf is the (familiar) uni-modal symmetric bell-shaped curve. The CDF Φ(x) is an elongated ess-shaped curve. The mean and variance of a normal distribution are the parameters µ and σ2. Many natural phenomena have normal distributions—physical measurements, astronomical variables etc.

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Descriptive Statistics.

Types of Data: We classify all ‘data’ about a variable into two types:

a. Categorical: data with ‘names’ as values:

Ex. 1 type of gamma ray burst GRB (short–hard, long-soft),

b. Numerical (or quantitative) data:: value is ‘numerical’

Ex.’s. mass of black holes, distance to stars, temperature at launch time of a shuttle, brightness of a star.

Numerical (also called quantitative) variables are divided into two types: discrete and continuous.

Parameters and Statistics.

Samples. When we obtain a sample from the population we also say we obtained a sample from the probability distribution.

Statistics are quantities calculated from samples.

Parameters are characteristics computed from the population as a whole or a probability distribution.

The quantities (, (, and (2 are parameters. Statistics are used to estimate parameters. For example, the sample mean is used to estimate the mean of the population from which the sample is obtained.

Graphical and Numerical Summaries of Quantitative Variables

Numerical Summaries:

1. Measures of Location:

Three commonly used measures of the center of a set of numerical values are the mean, median, and trimmed mean.

[pic]= Average of the data values,

Trimmed Mean: Delete a (fixed) proportion of smallest and largest observations (e.g., 5% or 10% each) and then re-calculate the mean. Judging in contests?

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Median, Arrange data in order from smallest to largest, with n observations.

If n is odd, the median is the middle number. If n is even, the median is the average of the middle two numbers.

Measures of Position in the Dataset:

The First Quartile Q1 is the median of the numbers below the median or the 25th percentile

The Third Quartile Q3 is the median of the numbers above the median or the 75th percentile.

Quantiles are order statistics expressed as fractions or proportions. For example, the pth quantile Qp(or 100pth percentile) divides the lower p of the data from the upper 1-p. For example, Q.67 (or 67th percentile) divides the lower .67 of the data from the upper .33 of the data. Q.25 and Q.75 are the first and third quartiles

The Interquartile Range (IQR) = Q3 – Q1

Five Number Summary: Min Q1 Median Q3 Max

Example 1. The body temperatures of 18 adults were measured, resulting in the following values:

98.2 97.8 99.0 98.6 98.2 97.8 98.4 99.7 98.2 97.4 97.6

98.4 98.0 99.2 98.6 97.1 97.2 98.5

Data Display (Sorted, from smallest to largest):

97.1 97.2 97.4 97.6 97.8 97.8 98.0 98.2 98.2 98.2 98.4

98.4 98.5 98.6 98.6 99.0 99.2 99.7

Descriptive Statistics: BodyTemp

Variable N Mean SE Mean StDev Minimum Q1 Median Q3 Maximum

BodyTemp 18 98.217 0.161 0.684 97.100 97.750 98.200 98.600 99.700

Five-Number Summary: Last five quantities in the descriptive statistics above:

Minimum Q1 Median Q3 Maximum

97.100 97.750 98.200 98.600 99.700

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A Boxplot (simple, no unusual observations) is a graphical display of the 5-# summary. The ‘box’ is drawn from Q1 to Q3 with the median shown in the box, lines are drawn from the minimum value to the bottom of the box (at Q1) and from the top of the box (at Q3) to the maximum value.

Outliers:

An observation is a mild outlier if it is more than 1.5 IQR’s below Q1 or 1.5 IQR’s above Q3. It is an extreme outlier if it is more than 3 IQR’s below Q1 or above Q3.

Software packages often identify outliers in some fashion; e.g., Minitab puts an ‘*’ for outliers (not necessarily all of them though).

Example. Number of CD’s owned by college students at Penn State University Stat students:

Variable N Mean SE Mean StDev Min Q1 Median Q3 Max

CDs 236 78.08 5.57 85.59 0 25 50.00 100 500

Mild Outliers: IQR = Q3 – Q1 = 100 – 25 = 75; (1.5)(IQR) = (1.5)(75) = 112.5. Mild Outliers are #CDs < 25- 112.5 or > 100 + 112.5 = 212.5. There are 17 values > 212.5 (multiples at some values)

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Extreme Outliers: 3IQR = (3)(75) = 225. Extreme outliers are #CDs < 25- 225 (negative value) or > 100 + 225 = 325. By this rule, there are several extreme outliers. See the boxplot below.

‘

Stem-and-Leaf Plots: A stem-and-leaf plot is a graphical display of data consisting of a stem--the most important part of a number--and leafs—the second most important part of a number.

Example. Stem and Leaf diagram of CDs:

Stem-and-leaf of CDs N = 236; Leaf Unit = 10

109 0 00000001111111111111111111111112222222222222222222222222222+

(56) 0 55555555555555555555555555555556666666667777777888899999

71 1 00000000000000000000000001122

42 1 555555555555555

27 2 00000000001

16 2 55555

11 3 000000

5 3 5

4 4 0

3 4 55

1 5 0

Resistant Statistics: A statistic is said to be ‘resistant’ if its value is relatively unaffected by outliers.

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Example 1. The salaries of employees in a small company are as follows:

$20K, $20K, $20K, $20K, $20K, $500K, and $800K.

The average salary is $200K. Delete the highest salary and find that the mean is $100K. Delete the two highest salaries and calculate the mean to be $20K. The median is $20 in both situations. The median is a resistant statistic and the average is not.

Example 2. Remove the 5 extreme outliers in the CDs dataset and redo the descriptive statistics.

Descriptive Statistics: CD’s with extreme outliers removed:

Variable N Mean SE Mean StDev Min Q1 Med Q3 Max

CDs(outliers out) 231 70.46 4.50 68.39 0 25 50 100 300

CDs outliers in) 236 78.08 5.57 85.59 0 25 50 100 500

Note that the only statistic in the 5%-number summary that changed was the Max (which had to change!) Note also that the mean decreased.

Examples. Resistant Statistics: Median, 1st and 3rd quartiles and IQR (for moderate samples—n = 10 or more roughly))

Non-Resistant Statistic: Mean (average)

Measures of Spread (Variability):

Interquartile Range IQR, Standard Deviation, Range = Maximum – Minimum, Mean Absolute Deviation, and Median Absolute Deviation.

The IQR measures the middle 50% of the data..

The Standard Deviation (SD) is roughly the average distance values are from the mean. The actual definition of the standard deviation (sd), denoted by s, is the square root of the sample variance s2, where

s2 = ∑ (xi - [pic] )2 / (n – 1)

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is the sum of squared deviations of the values from the mean.. The sd is not a resistant statistic

The mean absolute deviatio0n = (| xi - [pic]| / n

The median absolute deviatio0n = median [ | xi – median(xi)|]

Example. Body Temperature.

The interquartile range IQR = Q3 – Q1 = 98.600 - 97.750 = 0.850

The sample range = Max – Min = 99.7 – 97.1 = 2.60

The sample variance s2 = 0.467353; the SD = s = [pic]= 0.6836

The mean absolute deviation = 0.516667

The median absolute deviation = 0.400

Astronomy Example. We use data from Mukherjee, Feigelson, Babu, etal in “Three types of Gamma-Ray Bursts (The Astrophysical Journal, 508, pp 314-327, 1998), in which there 11 variables, including 2 measures of burst durations T50 and T90 (times in which 50% and 90% of the flux arrives) and total fluence (flu_tot) as the sum of 4 time integrated fluences. Descriptive Statistics for the Variables‘flu_tot’ and ln(flu_tot) are given below.

Variable N Mean SE Mean StDev

flu_tot 802 0.0000125 0.00000164 0.0000465

ln(flu_tot) 802 -12.955 0.0632 1.789

Five Number Summaries:

Variable Min Q1 Median Q3 Max

flu_tot .0000000159 .000000720 .00000234 .00000734 .000781

ln(flu_tot) -17.957 -14.144 -12.968 -11.823 -7.155

Empirical Rule says that if the data are symmetric and bell-shaped (unimodal), indicative of a normal distribution, then

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About 68% of the observations will be within 1 SD of the mean

About 95% of the observations will be within 2 SDs of the mean

Almost all—99.7%--of the observations will be within 3 SDs of the mean

For the variable ‘ln(flu_tot)’, we find that the intervals and the percentages are as follows:

Mean ( 1 StDev -12.955 ( 1.789 (-14.744, -11.166) 554/802 = 0.6908 or 69%

Mean ( 2 StDev -12.955 ( 3.578 (-16.533, -9.377) 763/802 = 0.9514 or 95%

Mean ( 3 StDev -12.955 ( 5.367 (-18.322, -7.588) 800/802 = 0.9975 or 99.75%

A similar dataset on gamma ray bursts included a categorical variable—gmark—with four values. A box plot graphically displaying this data for the variables log(flu)tot) is given below. It dramatically illustrates how transforming the data (here, using a log transformation) reduces or eliminates outliers, gives a visual comparison of the five #-statistics, and enables one to compare the values (median) four the four types of gamma ray bursts.

2. Coefficient of Variation = 100·StDev/|Mean| = (100)(1.789)/( 12.955) = 13.81. Small values of the coefficient are desirable (indicating small errors compared to the size of the observations).

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Standard Errors. The sample mean xbar is a statistic (a quantity calculated from a sample). As such, it varies from sample to sample and hence is a random variable with a probability distribution. It can be shown that

E([pic]) = ( = mean of the population from which the sample is taken

and

Standard deviation ([pic]) = (/(n

where ( is the standard deviation of the population from which the sample is taken and n is the size of the sample.

The standard error of the mean, denoted by se mean or s.e.(xbar) is the estimated standard deviation of Xbar:

s.e.( [pic]) = s/(n.

Empirical Cumulative Distribution Function

Definition: The empirical (cumulative distribution function (ecdf) is a step function given by

[pic], -∞ < x < ∞

where X1, X2, . . . , Xn is a random sample (from some distribution). An example is given below. In other words, the ECDF,[pic][pic] , increases by 1/n at each value in the sample (or k/n if there are k identical values at some value).

Example. Body Temperature. Here are the values of n = 18 body temperatures and a graph of the ECDF:

97.1 97.2 97.4 97.6 97.8

97.8 98.0 98.2 98.2 98.2

98.4 98.4 98.5 98.6 98.6

99.0 99.2 99.7

The ECDF e quals

0 for x < 97.1,

1/18 for 97.1 ≤ x < 97.2,

2/18 for 97.2 ≤x ................
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