15.871 - 15.874  15.871/874

As a reminder, we will only have the 10:00AM recitation tomorrow (in E51-325) and it may be shorter than the full period because there will be a second recitation before your final homework assignment is due.

We have placed the graded homeworks that we could not find a folder for next to John Sterman's office at E53-351.  Grading is still somewhat sporadic, though we are catching up and should have the majority of the assignments done soon.  

For the duration of the semester, 15.874 will only have the AM recitation (10:00 - 11:30). If you have conflicts with this, please let me know right away.

We are still seeing causal loop / stock and flow diagrams not following the basic rules. As a reminder, the most common issues are:

(1) missing polarity on arrow head,

(2) not closing important loops,

(3) not labeling loops appropriately with B/R and a name,

(4) causal links cannot influence a stock directly (only its flow), and

(4) units of stocks and flows need to be consistent.

So far, the class has done well on the first assignment. The TAs are grading diligently and we will let you know when we are finished. In the meantime, here are some common issues we would like to make clear:

B2.a. When members of the population are infected at the start of the simulation, we get "S" shaped growth (not just exponential).

B2.b. There is no mechanism to remove people from the infected stock of the simple SI model, so we will always infect our total population for allowable values of contact frequency.

B2.d. It is crucial to see that a false interpretation of the system might be concluded if too narrow a time horizon (such as 50 days) were used. One might conclude there is only exponential growth and not realize there is an effective carrying capacity.

C2.c. The single most important element missing from our system dynamics model is the behavior modification that reduces infectivity (mask use, etc.) and reduces contact frequency (quarantine, etc.) during the epidemic. Also note that SARSDATA shows "reported" cases and many early infections went undiagnosed for a short period.

C3. It is unreasonable to imagine a policy of "quarantine" in the case of HIV control. We were expecting comments on education and safe sex.