GLEASON: SOME APPLICATIONS OF THE QUADRAT METHOD 31 
Jaccard’s community coefficient (CC) was designed to express 
by a number the degree of similarity between two areas of vege- 
tation. His method is to divide the total number of species in 
both areas into the number common to both. Thus, two areas, 
each containing 30 species, of which 20 were common to the two, 
contain a total of 40 species, and the community coefficient is 
accordingly 50.. Jaccard’s method fails to take account of the 
much greater importance of some abundant species, and the 
resulting error of computation may be obviated, in part at least, 
by weighting each species with its frequency index. Referring 
to columns 5 and 6 of Taste I, and weighting each species with 
its average index in both columns, the results, shown in TABLE IT 
TABLE II 
In first Common to} In second 
area only | both areas | area only 
Pieris MONUENG os es eee sper 99 
Va ae oo Po eee te cee 80 
Gaylus. Wha oe Cs a a 42.5 
Aster a. 2y tat 1 OPERA AR AES ap bye ee op een URC TORRENS MR 33-5 
Re ENE sis ins a pow eS 38.5 
ly ee PTE Pine igre tip yarns non 24 
Dees AMisdlg 8. ee 20 
MMOS Misghinionsic. | 2.3020. ask A 26 
Panicum POM ee ki oe ee epee 8.5 
Wnmonia sPicdia) cs. Bee 2 
Goultheria procumbens... ... 0.0.0 .0cccusususeces 13.5 
Gnophalium decurrens. .... 0... 600. cde soe: It 
P Ofulus grandidentaia.®... onc os. 5 
Mierariunm tnosum......... oe 5 
Erigeron PENGUINS Se Oe a ee ae 2.5 
Panicum FOMLRODNY SUM 6 oo Sa a I 
MOO LONE. oe 7-5. 
Series theadseels. oe I 
SNE Dromalis: F205. ee 4-5 
Rachich conadensis. 26 hfs, ae 15 
Poa TO POMOU ORE, ee eg ee 2.5 
Quercus oce.ng PRT Ee Menie  ee Rn ORI ar I 
WG Winbeliata i : 1.5 
Amelanchier canadensis ew TR Geos ee I 
Mieke oe ae 
13.5 433-5 8.5 
are obtained, from which the community coefficient of the two 
areas concerned is determined as 95, by dividing the sum of the 
three columns, 455.5, into the sum of the center column. 
By Jaccard’s method, giving equal weight to each species, 24 
total species divided into 15 common species = CC 62. 
