\reviewtitle{The Back of the Envelope and The Envelope is Back}
\reviewlabel{bentley84envelope}
\reviewauthor{Jon Bentley}

Spending a few minutes to half-an-hour writing down and working out
roughly how something will perform can be an enormous time saver and
can help you from making bad decisions.  Bentley says the envelope
can be used to compare two approaches to a problem or to see if a
particular solution is feasible at all.  There are two main keys in
performing a back of the envelope calculation:
\begin{enumerate}
\item
Employ a safety factor to compensate for what you don't know.  For
example, Roebling made the Brooklyn Bridge six times more strong than
his calculations had led him to believe, because he knew that he
didn't know everything about the situation.
\item
Like Einstein said, ``Everything should be made as simple as
possible, but no simpler.''  Don't make your calculations too simple!
\end{enumerate}

Bentley goes on to supply a bunch of problems and solutions.
\begin{enumerate}
In ``The Envelope is Back'' Bentley focuses on the following:
\item
orders of magnitude.  Be wary of the difference between microseconds
and milliseconds, even though they both seem small to you.
\item 
Two answers are better than one.  If you can do two back of the
envelope calculations in two different ways and they both agree, you
can be far more confident in your answer.
\item
Make sure that your dimensions agree, \eg that you are adding feet
and feet and that the dimension of a product is the product of the dimensions.
\end{enumerate}

He then talks briefly about queues.  Little's Law states that ``The
average number of things in the system is the product of the average
time each on spends in the system.''  (And if there is a gross ``flow
balance'' of things entering and leaving, the exit rate is also the
entry rate.)