Materials for Lecture 08 Chapters 4 and 5 Chapter 16 Sections Lecture 08 Bernoulli & Empirical.xls Lecture 08 Normality Test.xls Lecture 08 Parameter Est.xls Lecture 08 Normal.xls Lecture 08 Simulate a Reg Model.xls

Stochastic Simulation Purpose of simulation is to estimate the unknown probability distribution for a KOV so decision makers can make a better decision –Simulate because we can not observe and measure the KOV distribution directly –Want to test alternative values for control variables Sample PDFs for random variables, calculate values of KOV for many iterations Record KOV Analyze KOV distribution

Stochastic Variables Any variable the decision maker can not control is thought to be stochastic In agriculture we think of yield as stochastic as it is subject to weather For most businesses the prices of inputs and outputs are not directly controlled by management so they are stochastic. –Production may be random as well. Include the most important stochastic variables in simulation models –Your model can not include all random variables

Stochastic Simulation In economics we use simulation because we can not experiment on live subjects, a business or the economy without injury In other fields they can fabricate an experiment –Health sciences they feed/treat multiple rats on different chemicals –Animal science feed multiple pens of steers, chickens, cows, etc. –Engineers run a motor under different controlled situations (temp, RPMs, lubricants, fuel mixes) – Vets treat different pens of animals with different meds –Agronomists set up randomized block treatments for a particular seed variety All of these are just different iterations of “models”

Iterations, How Many are Enough? Change the number of iterations based on nature of the problem is adequate. − Some studies use 1,000’s because they are using a Monte Carlo sampling procedure which is less precise than Latin hypercube −Simetar uses a Latin hypercube so 500 is an adequate sample size Specify the number of iterations in the Simetar simulation engine Specify the output variables’ names and location

Normal distribution a continuous distribution that produces a bell shaped distribution with set probabilities Parameters are –Mean –Standard Deviation Normal distribution reaches to + and - infinity. –Can produce negative values so be careful –Can produce extremely high values Most of us have memorized several probabilities for the normal distribution: –66% of observation within +/- 1  of the mean –95% of observation within +/- 2  of the mean –50% of observations lie above and below the mean. Normal Distribution

Normal distribution is used frequently, particularly when simulating a regression model Parameters for a Normal distribution –Mean expressed as Ῡ or Ŷ –Standard Deviation σ (or SEP from a regression model) Assume yield is a random variable and have production function data, such as: –Ỹ = a + b 1 Fert + b 2 Water + ẽ –Deterministic component is: a + b 1 Fert + b 2 Water –Stochastic component is: ẽ Stochastic component, ẽ, is assumed to be distributed Normal –Mean of zero –Standard deviation of σ e See Lecture 8 Simulate a Reg Model.XLS Simulating Random Variables

PDF and CDF for a Normal Dist. f(x)F(x) Probability Density FunctionCumulative Distribution Function -- ++ ++ --

Use the Normal Distribution When: Use the Normal distribution if you have lots of observations and have tested for normality Watch for infeasible values from a Normal distribution (negative yields and prices)

Problems with the Normal It is easy to use, so it often used when it is not appropriate It does not allow for extreme events (BS’s) –No way to account for record breaking outliers because the distribution is defined by Mean and Std Dev. Std Dev is the “average” deviation from the mean and averages out BS’s Market outliers are washed away in the average It is the foundation for Sigma 6 –So it suffers from all of the problems of the Normal –Creates a false sense of security because it never sees a record braking outlier

Test for Normality Simetar provides an easy to use procedure for testing Normality that includes: –S-W – Shapiro-Wilks –A-D – Anderson-Darling –CvM – Cramer-von Mises –K-S – Kolmogornov-Smiroff –Chi-Squared Simetar’s Hypothesis Testing Icon (Ho Hi) provides a tab to “Test for Normality”

Normal Distribution =NORM( Mean, Standard Deviation) =NORM( 10,3) =NORM( A1, A2) Standard Normal Deviate (SND) =NORM(0,1) or =NORM() SND is the Z-score for a standard normal distribution allowing you to simulate any Normal distribution SND is used as follows: Ỹ = Mean + Standard Deviation*NORM(0,1) Ỹ = Mean + Standard Deviation*SND Ỹ = A1 + (A2 * A3) where a SND is in cell A3 Simulating a Normal Distribution

General formula for the Truncated Normal =TNORM( Mean, Std Dev, [Min], [Max],[USD] ) Truncated Downside only =TNORM( 10, 3, 5) Truncated Upside only =TNORM( 10, 3,, 15) Truncated Both ends =TNORM( 10, 3, 5, 15) Truncated both ends with a USD in general form =TNORM( 10, 3, 5, 15, USD) Truncated Normal Distribution

Example Model of Net Returns for a Business Model -Stochastic Variables -- Yield and Price -Management Variables -- Acreage and Costs (fixed and variable) -KOV -- Net Returns -Write out the equations and exogenous values Equations and their order

Program a Simulation Model in Excel/Simetar  -- Input Data Section of the Worksheet 1VC / acre VC / Y0.25 3Acre100 4Fixed Cost 10 5Yield Mean & Std. Dev See Lecture 08 Simulation Model with Simetar.XLS ACB Price Mean & Std. Dev

Program Model in Excel/Simetar  -- Generate Random Variables and Simulate NR 13Stochastic Yield Formulas in Column B 14 Mean150= B5 15 Std. Dev.30= C5 16 SND0.362= NORM ( ) 17 Random Yield160.86= B14 + B15 * B16 18Stochastic Price 19 Mean2.00= B6 20 Std. Dev.0.40= C6 21 SND-0.216= NORM ( ) 22 Random Price1.9136= B19 + B20 * B21 23Receipts from Market 24 Yield160.86= B17 25 Price1.9136= B22 26 Acres100= B3 27 Receipts = B24 * B25 * B Calculate Costs 30 Fixed Cost10= B4 31 VC/acre4000= B1 * B3 32 VC/Y2412.9= B2 * B17 * B4 33 Total6422.9= Sum (B30 : B32) ABC Net Returns = B27 – B33

1X PDF for Bernoulli B(0.75) X CDF for Bernoulli B(0.75) PDF and CDF for a Bernoulli Distribution. Bernoulli Distribution Parameter is ‘p’ or the probability that the variable is 1 or TRUE Simulate Bernoulli in Simetar as = Bernoulli(p) = Bernoulli(0.25)

Use Bernoulli in a conditional distribution as demonstrated: –It rains 20% of time during June and if it rains, the amount is distributed U(0.1, 0.9) Cell A2 =BERNOULLI(0.20) Cell A3 =UNIFORM(0.1, 0.9) * A2 –Probability of mechanical failure is 5%, cost of repair is $10,000, $20,000, or $30,000 Cell A4 =BERNOULLI(0.050) Cell A5 =DEMPIRICAL(10000, 20000, 30000) Cell A6 = A4 * A5 Bernoulli Distribution