SCLS Delivery Volume Estimate Methodology
For the majority of SCLS member libraries, yearly delivery volume is determined by counting the number of red baskets sent and received each day of the week during three sample periods each year (typically in March, July, and November). SCLS Delivery then multiplies the total number of baskets by a "per-basket item average" to estimate item delivery volume for the sample week. The volume estimates from the three samples are averaged to create a weekly volume estimate, which is multiplied by 52 weeks to estimate the total volume for the year.
The Per-Basket Average
Prior to 2002, delivery volume at all SCLS libraries was determined by hand-counting all items picked-up and dropped-off at each library during our sample weeks. In 2002, SCLS Delivery instituted the per-basket item average system in order to cope with increased delivery volume. Libraries that have a normal delivery volume of two or more red baskets are counted using this system. Based on a large sample of hand-counted baskets, it was determined that a typical basket for an SCLS library contained 34.37 items.
Changes in circulation trends, notably in AV formats and sizes of items circulated, led to a recalibration of the per-basket average in early 2009. Over a week's time, a representative sample of hand-counted baskets from various SCLS libraries yielded a new per-basket average of 41.06 items.
2009 yearly volume estimates reflect the first use of current, higher average which accounts for the drastic increase in volume for most libraries betweeen 2008 and 2009.
For future volume estimates, SCLS Delivery plans to re-sample the per-basket item average every two to three years to ensure our statistics more accurately indicate the volume we handle in support of SCLS resource sharing and better reflect growth trends.
Detailed volume statistics, including statistics for individual libraries and/or particular sample periods, are available upon request.