The Museum of HP Calculators


Erlang C Calculator for the HP-33E

This program is by Bill Harrington and is used here by permission.

This program is supplied without representation or warranty of any kind. Bill Harrington and The Museum of HP Calculators therefore assume no responsibility and shall have no liability, consequential or otherwise, of any kind arising from the use of this program material or any part thereof.

Overview

Erlang C is used to calculate the number of call centre agents required to provide a particular level of service at a particular call load. For example, how many agents are required to handle a peak hour call load of 1000 calls if the average call duration is 60 seconds and callers are to wait no more that 30 seconds 80% of the time?

Listing

/* Start Manual Initialise (see below)*/
Enter peak hr calls then R/S
1	/* Line 01*/
STO + 6	/* N */
RCL 3	/* A */
RCL 6
yx
RCL 6
Enter	/* x! */
Enter
1	/* Line 09 */
-
x = 0
GTO 16
*
Last x
GTO 09
Rv	/* Line16, n! */
/
STO + 5	/* Sum(AN/N!) */
RCL 5
/
STO 7	/* E1,N (A) */
RCL 6
*
RCL 6
RCL 3
-
RCL 3
RCL 7
*
+
/
STO 4	/* E2,N (A) */
RCL 3
RCL 6
-
RCL 1
*
RCL 0
/
ex
*
CHS
1
+	/* Svc Level Met */
RCL 2	/* SLA */
x > y	/*If SLA not met */
GTO 01	/* Loop */
RCL 6
GTO 00	/* # Agents */

<Manual Steps - because I've used all the program memory>
Rv	/* Roll down */
Rv
<Result>	/* Svc Level Met this proportion of the time*/
RCL 4
RCL 6
RCL 3
-
/
RCL 0
*
<Result>	/* Av wait time in seconds*/

Set up (for each new call centre SLA):
Call duration in secs STO 0
Svcs Lvl Answer time (secs) STO 1
SLA (eg. 0.8 for 80% of time) STO 2
Traffic (A), (ie.Calls/3600*M0) STO 3

Initialise (for each new call volume):
0 -> STO 6, STO 4; 1 -> STO 5
Type peak hr call qty, then R/S

Other values:
Traffic Intensity (A) is in M3
Erlang B is in M7
Erlang C is in M4

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