The Limits of Analysis
 or 
Some fictions are more ridiculous than others
Men in general are more affected by what a
thing appears to be than by what it is, and are
frequently influenced more by appearances than
by the reality.
 Machiavelli
An account is best described by the amount and timing of its flows.
Only accounts with flows at equal periods, at a constant rate, are well
described by a single number  typically, savings and loan accounts. All
other accounts, including simple interest accounts with long periods (simple
interest is the basic building block of compound interest accounts), are not
well described by a single number.
I=PRT's logically dubious method, using timegeometric, can extract an annual
rate from a simple interest rate with a period longer than one year.
Discounted cash flow [DCF], using timeexponential, can extract an annual rate
from a simple interest rate that has a period longer than one year. Do it at
your own risk: a simple interest account's flows and timing better describe
the account.
DCF treats simple interest as a twoflow problem, an
inflow and an outflow. DCF
can provide an annual rate (though a rate can be
calculated on any timebase), for theoretical flows, at
timebasetime, compounded annually. The problem with a
DCF analysis of simple interest is that simple interest
has no flows at timebasetime. You can't dine out
on theoretical flows.
A discounted cash flow twoflow annualrate extraction:
 Given:
 I=PR, a simple interest account (i. e., one period)
 $100 principal
 10year period
 100% rate
 Discounted Cash Flow, 2 flows (spreadsheet notation)
 APY = 100*((outFlow/inFlow)^(timebase/timeBetweenFlows)1)
 APY = 100*((200/100)^(365/3650)1) = 7.177346
 Reverseengineer the DCF (compound interest):
 inFlow*(1+%RateAnnual/100)^(time/timebase) = outFlow
 $100*(1+7.177346/100)^(3650/365) = $200
 Notes:
 1. A rate can be stated on any timebase.
 2. Time and timebase must be stated in the same units.
Just because you can reverseengineer a simple interest rate to an annualized
compound rate, that doesn't mean that you have a useful description of the
simple interest rate account. The real flows for $100 at 100% interest,
10years, are very different from the hypothetical flows for $100 at 7.177346%
interest compounded annually. There is no compounding in a simple
interest account. The 7.177346% rate is a simulation, not a representation, of
reality.
Men have become tools of their tools.  Thoreau
