 I can briefly introduce you to Vensim, Vensim is a software we can use to simulate system dynamic models. If you put the underline equation and click the run icon, there is one shown here, guy running it will run the simulation. If you do not put the equation, you can still draw some nice diagrams like you know the causal loop diagram etcetera, there are nice arrows etcetera it can help you do that. This is one of the most simplest interfaces I have seen, though there are many icons, the icons of interest are what is highlighted there. So, there is a fourth icon from the left 1, 2, 3 forget it nothing, this is just lock screen, this is just to move things around. This one which has a pencil and an alphabet VAR I think, VAR we use it to define auxiliary variable and there is a pencil with a rectangle with VAR written that is to create a stock and then one arrow link to create a link that is a causal link and a double headed arrow with a valve symbol that you just saw whatever we have been using the same thing is used to define a rate link. That is exactly what we need, we do not need anything else all other things are for just visualizing deleting and things like that, this is like the pacman thing to delete not needed. So, this is entire thing you will ever be using for building models, apart from Wensim there are other softwares like I think, Powersim, softwares I think I am not thinking I think Powersim any logic etcetera. So, the symbols may be slightly different, but you just look for they use a variable rate some may use the rate term flow rate some may use the term level instead of stock, but usually they have a symbol in a somewhat rectangle out there in so many ways only you can draw a rectangle. So, you will have that double headed arrow roughly be the same with a valve symbol maybe they get more graphic with the symbol of the valve, but pretty much this one and once you simulate we will be using the icons on the left side to visualize the output you just click the variable click that graph symbol you will get a graph immediately and if you. So, these first two are for the graphs the next two are for actually seeing the values that are simulated. So, once you get the actual values you can see and understand what is the computation happening or sometimes you will get to out at simulation, because this is just just numerical integration nothing else it just just numerical integration. So, sometimes it will say out of bounds or values too high or value crosses 0. So, then you can actually see the numbers and see where it happened and what is happening and things that or you can copy paste it into excel to get better graphs out of system. So, there is a free software the student version is free you can download it from vensem.com you have to go register yourself give your email id they will send you a link then you download the software. So, we will be using this in the in this course this is a vensem interface you click download and go to free download go for the vensem PLE it works only on Windows and Mac if you have Linux laptop install wine it works fine in wine no problem yeah it is full the details click download it in the software and that is sufficient for our course purposes looks like I downloaded a slightly new one see here already it is different from the picture I showed in the slide and apologies for that explicit says variable level arrow rate better symbols. So, we will just go with that simulators the play button here and left side these are all the images looks like the pictures of a older version of vensem. So, this is more clearer the first we do is when we create a new model the only thing it has is initial time final time time step and what is the general time units that we are looking at let us say we are going to model this interest rate example. So, we want to keep the say time units as here and say time step as one final time there it was 15. So, let us let us take it 20 time units to the time step of one click ok you could just a blank screen. So, we had one rate one flow say same thing one rate one stock and auxiliary variable. So, first always put the stock it is easier I will tell you why and then you put a rate solid it was I did not draw anything I just clicked it somewhere and just wrote the name it gave the rectangle there is no right click left click just one click and then you click rate then you click we wanted to go the direction. So, start point to end point start is outside the system you click somewhere there is no I am not clicking and dragging then you just move it and somewhere inside the stock again make one more click then it will ask the name of the variable net let us call net income and you got this stock diagonal. If you do not click it inside and if you click it somewhere outside then the cloud will appear on both ends like this rate if I click it here and click it here and so called it then I get cloud on both sides. So, sometimes we happen this and suppose it goes very here and click it ok it is connected but sometimes it may not connect I am just going to delete this all these it is a delete button and I want one variable that was interest rate no oversize we are moving it here arrows again same thing I click inside the variable name and to get a curved arrow it has a nice feature you click it somewhere in the white area and then you click the next variable name it will give you a curved arrow principle then again white area then I do this I get a nice curved arrows you will have a specific exercise for that next week so you just observe. Then we give values for each equations you have to click equations here then everything in black means you have not given any equation further. So, let us go to principle it already shows integration of net income I am already integrating net income I have nothing else to add there initial value we told is 100 the main thing we need to write here is what is the units so that I can do the units check. So, units here is let us define it as rupees RS so that is a unit there is only everything added here and added initial value 100 I click ok this became white it means some equation is put where the equation is right or wrong it do not tell there is something written there as part of the equation and let us go to interest rate interest rate is a constant value. So, let us just put it as constant 0.15 and units is 1 by here time unit is here the fraction thing is just 1 over here and I go to net income and in an equations I have to multiply interest rate into principle. So, on the big box here you see all the variables which is flowing into the this variable which is connected to this variable. So, you need to form in this case interest rate and principle both the values are coming to the net income. So, I need to form an equation considering both suppose I skip 1 I just put interest rate it will say following input is not used but was expected. So, then you know you have a causal link then you can again go back and then multiply it added and units of this will be rupees per year ok. So, that is it then if you go to model say units check save it all units are ok good then you go to model check model model is ok as it will show there is a type mismatch units mismatch or some variables connected you forgot the equation or some variables defined but it is not connected to anything else whatever errors it will just show. So, once we do that we just click the simulate button I just simulated it it is simulated it is done it does not take so it is soul hung to simulate if you want you can simulate it again over in the data set. So, in principle now I want to see what is happening I just clicked it then it becomes whatever is selected it will be black right because this is what I want I click graph and I see the system nothing as I resize it it automatically rescales it for you or some legends come. If you want to see the underlying values then I click this one then I can exactly see the values or it is being completed right. I can look at other graph also net income this is an interesting thing suppose I click this one and I click causes strip it shows how that variable changes along with all the variables which are acting as input to this stock is only affected by the flow so by one flow in this case. So, that stock and how that corresponding flow is happening so sometimes it is useful to see all these things together so that you can compare and we can create much more complicated graphs that even if it is not connected you can select them as you can visualize it we will learn some of those features later for modeling per se that is it it is not too difficult people can do it difficult is finding the equation but drawing it per se it is as simple as what you thought.