anchor.diff:  Performance metrics statistics
   ------------------------------
   
  -f(t) (e.g. response time, concurrency)
  -
  -* Average
  -* Min, Max
  -* Percentile
  -
  -f(t, sampling) (calculated, e.g. throughput, cumulated time)
  -
  -* overall: sampling period = whole test duration
  -* idem f(t)
  +*Instant measures*
  +
  +** Average
  +** Min, Max
  +** Percentile: Pct~n~ = X | n% of the data is <=X{br}
  +(Note that median &equiv; Pct50)
  +
  +Statistics can produce:
  +
  +* a scalar, when calculated for the whole duration of the test
  +* f(t), when calculated for each fixed interval &Delta;t
  +
  +*Interval measures*
  +
  +** only one value per interval &Delta;t -> derived 'instant' mesure, with lower resolution
  +** it's possible to calculate statistics on this derived measure
   
   
   Measure window
  
positive.code: 
  Measures of the UoT (&ne; measures/monitoring of the Test Harness)
  
  [horizontal]
  Response time:: (time response is received) - (time request was sent){br}
    _! the timestamp given in the logs may be either the time of request or the time of response,
    depending on the system_
  Throughput:: &Delta;n/&Delta;t; where n = number of transactions
  Concurrency:: count of concurrent requests at a given time{br}
    _(difficult to compute precisely: sort all timestamps (request *and* response), then travel the list while incrementing (request) / decrementing (response) a counter_
  
  
negative.code: Performance metrics (cont'd)
  ----------------------------
  
  Interval measures: f(t, &Delta;t)
  
  [horizontal]
  Throughput:: &Delta;n/&Delta;t; where n = number of transactions
  Cumulated time:: &Sigma; response-time / &Delta;t
  Workload:: &Delta;work/&Delta;t; where work is a measure of the work accomplished (e.g. # records, file size...)
  
  Others
  
  [horizontal]
  Capacity:: how much workload the system can absorb
  Availability:: amount of time system is available / &Delta;t
  Scalability:: ability of the system to utilize more (or less) hardware to match variations of the workload (!up &ne; down)
  