Need help with formulas

[liizz]

Hi all,

I need help with my formulas..

@Tony: I have tried really hard to solve some formula problems but still there are some issues in the model..

I never failed in programming assignments but I see a failure coming for me in SD.

I really need help..

Thanks
Liizz

[Administrator]

We are more than happy to help. But we will not build or debug a model for you.

You haven't said in this post what you need help with. What formula are you struggling with, and have you checked the Vensim help system? It contains many sample models.

[liizz]

Hi,

Thanks.

Of course I never expected you to design my model. I always ask for suggestion or advice before because I do not have a strong exposure in system dynamics and at times i'm uncertain about my point.
Doing an introductory module on System Dynamics at the college will not make me a genius in the field and personally, it takes time to have a system dynamic thinking.

I have written an equation for the Project perceived as completed based on Tasks Done Correctly and Scheduled Completion date.
IF THEN ELSE (Tasks Done Correctly > Initial Tasks,0,1) where Initial Tasks is 2000.

But since the project is not yet finished, I am unable to increase workforce.
Formula : Extra Personnel Needed=
MAX(MIN(Max Hires,Required workforce)-Cumulative Workforce Level,0)

Thanks
Liizz

[lorenzo.cioni]

Hi,

Thanks.

Of course I never expected you to design my model. I always ask for suggestion or advice before because I do not have a strong exposure in system dynamics and at times i'm uncertain about my point.
Doing an introductory module on System Dynamics at the college will not make me a genius in the field and personally, it takes time to have a system dynamic thinking.

I have written an equation for the Project perceived as completed based on Tasks Done Correctly and Scheduled Completion date.
IF THEN ELSE (Tasks Done Correctly > Initial Tasks,0,1) where Initial Tasks is 2000.

But since the project is not yet finished, I am unable to increase workforce.
Formula : Extra Personnel Needed=
MAX(MIN(Max Hires,Required workforce)-Cumulative Workforce Level,0)

Thanks
Liizz

Hi
in your post you mention some strange variables (such as Scheduled Completion date and those in MAX(MIN...))) and moreover you do not say anything about the reasons why your IF statement gives you troubles....
Cheers
Lorenzo

[liizz]

Tony, is there an e-mail address where I can send you my model.

Thanks
Liizz

[liizz]

Please find model

[Administrator]

But since the project is not yet finished, I am unable to increase workforce.

Why? You need to spend the time and look at the model and trace back the reason why you cannot increase the workforce. Is it because of input data? Or an incorrect equation somewhere?

[liizz]

Yes.. Thanks for the help.

[tomfid]

It's often helpful to isolate the sectors of the model (for example, rework cycle and personnel skills), so that they can be tested independently and refined before they are integrated.

[liizz]

Hi,

What you mean by isolating? Should i start one afresh?

Thanks
Liizz

[liizz]

Tom,

Is it possible to get some discussion with you? I must say that I was able to start my model thanks 2 you as it makes me see things differently.

Thanks
Liizz

[liizz]

Infact, I started designing my module one by one. But when I integrate it, big issues coming out

Thanks
Liizz

[tomfid]

There are two ways to isolate:
- copy/paste the structure into a separate model, improve it until you are satisfied, then copy/paste back into the complete model, or
- break the causal links between the elements of the integrated model temporarily

One way to implement the second approach might be:

Code: Select all

{personnel sector}
staff = INTEG( hiring-firing, initial staff )
{task sector}
work accomplished = effective staff*productivity
effective staff = IF THEN ELSE( staff test input, constant staff, staff )
constant staff = 10
staff test input = 0 or 1

[liizz]

Hi all,

The hiring process is working well in stand-alone mode but when coupled with productivity. Getting issues on that.

Am discouraged, am struggling too much on that.

Can I get some serious assistance on that plz?

[Administrator]

You need to work out why it isn't working. Spend some time using the Vensim table tool and look at the numbers to work out why it's not doing what you want.

[liizz]

Tony,

I have been able to troubleshoot some errors up till now but this one is taking too much time.

I am making use of graphs itself. [Please see file]

The logic of the equation for the hiring process is looking fine.

My equation for : Effect of experience on productivity=
IF THEN ELSE(Cumulative Workforce Level > 0,(New Personnel*Normal productivity of new personnel
+Experienced Personnel)/Cumulative Workforce Level,1)

Thanks to advise
Liizz

[Administrator]

I have been able to troubleshoot some errors up till now but this one is taking too much time.

Sometimes debugging models takes a long time. Unfortunately it is something you need to do, you cannot expect others to debug your model for you. We can help with technical problems or general approaches for small problems, but not debug a model the size of this one.

I am making use of graphs itself.

Use the table tool.

The logic of the equation for the hiring process is looking fine.

My equation for : Effect of experience on productivity=
IF THEN ELSE(Cumulative Workforce Level > 0,(New Personnel*Normal productivity of new personnel
+Experienced Personnel)/Cumulative Workforce Level,1)

Thanks to advise
Liizz

What do you need advice with?

Looking at the graphs in the file, what does "effect of experience on productivity" actually mean? If it means as experience rises, productivity rises, then you model logic must be incorrect as the line is decreasing.

[Administrator]

I have been able to troubleshoot some errors up till now but this one is taking too much time.

Sometimes debugging models takes a long time. Unfortunately it is something you need to do, you cannot expect others to debug your model for you. We can help with technical problems or general approaches for small problems, but not debug a model the size of this one.

I am making use of graphs itself.

Use the table tool as well. When debugging models, you need to look at values as well as graphs to check it is doing what you expect it to.

The logic of the equation for the hiring process is looking fine.

My equation for : Effect of experience on productivity=
IF THEN ELSE(Cumulative Workforce Level > 0,(New Personnel*Normal productivity of new personnel
+Experienced Personnel)/Cumulative Workforce Level,1)

Thanks to advise
Liizz

What do you need advice with?

Looking at the graphs in the file, what does "effect of experience on productivity" actually mean? If it means as experience rises, productivity rises, then you model logic must be incorrect as the line is decreasing.

[liizz]

Effect of experience on productivity is based on the no. of either experienced or inexperienced personnel I have?

Lets say I have 10 inexperienced, the graph shall show a decline and for experienced people it shall be a rise.

Can I send u a simplified version of it?

Thanks
Liizz

[Administrator]

Lets say I have 10 inexperienced, the graph shall show a decline and for experienced people it shall be a rise.

Is that what happens in your model? Put 10 inexperienced people in and see what happens. Part of model building is experimenting yourself to determine if your model is behaving correctly. This is something you need to do yourself, or discuss with your tutor.

[liizz]

Yes as per the graph i sent you, for inexperienced its declining.

But for experienced, it is below 2 and is going nearly constant.

haha, my tutor has no experience in modelling and no discussion

Thanks
Liizz

[liizz]

Plz can I send u a simplified version of my model?

Thanks
Liizz

[liizz]

Kindly find the model.

Thanks
Liizz

[Administrator]

Yes as per the graph i sent you, for inexperienced its declining.

But for experienced, it is below 2 and is going nearly constant.

haha, my tutor has no experience in modelling and no discussion

Have you traced through why these values are being calculated?

[tomfid]

Take a look at http://vensim.com/causal-tracing-in-vensim/, then revisit your model.

The aggregate behavior looks pretty reasonable to me, as long as willingness to increase workforce = 1. There's some steady state error (equilibrium workforce is short .3 people), because the hiring decision doesn't anticipate personnel leaving. That could be realistic, or you could add a term to the hiring rate to account for it.

The distribution of staff between inexperienced and experienced seems incorrect. If the learning and retention delays are both 10 months, you'd expect both stocks to have about the same number of people in them. I think this relates to the lookup used, which is not serving the purpose that I would guess is intended (arrival distribution for experience). Get rid of the lookup and use the standard stock/delay formulation for the assimilation rate and it should work fine.