A note on a modification of the spread of participation index allowing for unequal zones

The spread of participation index (SPI) is used to indicate the extent of enclosure utilisation by captive animals. The established method of determining the SPI requires the division of the enclosure into equal sized zones. This method has several practical and theoretical difficulties, most importantly that the SPI is hugely affected by zone size, for which there is no a priori method to determine. Thus it is desirable to develop a method to determine the SPI allowing for unequal zones which can be assigned on the basis of enclosure resources. Such a method is described in this paper and compared with the original method in a variety of hypothetical situations. The modified SPI is the exact mathematical equivalent of the original SPI if zone sizes are equal. In a range of realistic situations the modified SPI is more sensitive and more accurately reflects the extent of enclosure utilisation. (C) 2003 Elsevier B.V. All rights reserved.