> Hello Mark, > Regarding the homework assignment due Friday, I was able to make the > figure correctly, but cannot find the correct equations to use. I read > the 12 pages that were assigned and I used the information found on > there, but my Vensim model is still having problems. > Where or how can I find the correct equations? > > Thank you, > Frodo Baggins > MWF 12-12:50 > Hello Frodo, Thanks for the Wednesday morning question. And I found another email note from an 8 am student that was sent on Thursday evening with the same question. He mentioned other students not knowing where the equations were either, and then on Friday morning during class nobody asked about it, and unfortunately I had not seen the email note yet. So, here is where the equations are: http://www.cs.uni.edu/~jacobson/025/VensimAssignOneModel.jpg is at the bottom of this web page, and is a portion of PAGE 5 of chapter 02 of the Andrew Ford book. Right there by Figure 2.12 on page 5 is all the information you need for the equations. http://www.cs.uni.edu/~jacobson/025/vensimAssign01F16.html Read pages 1 through 5 of the Andrew Ford book to review the population model that we created that only had BIRTHS, then we made it a little more complicated by having BOTH BIRTHS and DEATHS. http://public.wsu.edu/~forda/Ch%202.pdf Page 5 of that ANDREW FORD book is where you get the additional EQUATIONS and constants data needed to make the assignment population model. It states: 1. Fraction FEMALE is 0.5 or 50% of the population 2. Fraction Female that are biologically mature is 36% or 0.36 would be the EQUATION. 3. If you know the POPULATION and you know the FRACTION FEMALE, what would be the equation to use to find out FEMALES population? Suppose you have a population of 100 (million) and the FRACTION FEMALE is 0.5. How many FEMALES do you have? 50 million of course. 100 * 0.5 gives 50. The equation is found by just thinking with a concrete example. Look at the graph. The incoming arrows are for TWO THINGS: Fraction Female and POPULATION 0.5 100 for example What is the output? Fraction Female * POPULATION would give 50. 4. How many MATURE FEMALES are there? 0.36 is the FRACTION OF FEMALES THAT ARE BIOLOGICALLY MATURE 50 (million) is the number of females if the population is currently 100 (million) and fraction female is 0.5. So how many MATURE FEMALES? 0.36 times 50 = 18 (million) MATURE FEMALES. Multiply the one variable times the other variable. Look at the diagram. So it would be 18 million mature females when the POPULATION is 100 million. 5. How many births that year? It says we assume a mature female gives birth every other year, so 0.5 births per year is the number of births per mature female per year. So if we have 18 million mature females in our population of 100 million, we multiply the 0.5 births per year times the 18 million mature females and get 9 million babies born. 6. AVERAGE LIFETIME is says we should use 50 for the equation. 100 (million) POPULATION divided by 50 gives us 2 (million) deaths. "The equation for deaths will be the population divided by the average life time." That quote is right there on page 5, and on the JPG at the bottom of the page for page 5 excerpt. 1 Note that dividing by 50 is the same as multiplying by -----, right? 50 So dividing the POPULATION by AVERAGE LIFETIME (50) is exactly the same as MULTIPLYING POPULATION by 0.02, since 1/50 = 0.02. And basically, a 2 percent death rate that was used in the previous simpler models would be the equivalent of 1/50th of the population dying each year, which corresponds exactly to an average lifetime of 50 years. http://www.cs.uni.edu/~jacobson/025/VensimAssignOneModel.jpg http://www.cs.uni.edu/~jacobson/025/vensimAssign01F16.html Reread pages 1 through 5 again for the whole process for all of the equations needed. Take notes on a separate sheet of paper as you scratch out what you think the equations would be. Good luck! Mark