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READING RATES 499 



PRINCIPAL EXPERIMENTS 



Experiment 1 : Effect of Vocabulary Size 



The larger the vocabulary size the higher the information rate con- 

 veyed by a given word reading rate. However, one might think that it 

 would be possible to read randomized lists of, say, 4 words substantially 

 faster than lists of 8, or 16 or more words.* How is the word rate affected 

 as the vocabulary size increases? 



To in\^estigate vocabulary size as such, it is necessary as far as possible 

 to avoid the influence of differences in word length or familiarity. To 

 this end, words in each vocabulary were chosen at random from the 500 

 most common words in the language ;^ a few words were then changed so 

 as to keep an average of 1.5 syllables/ word for each vocabulary. Figs. 

 1(a) and 1(b) show parts of typical lists for vocabulary sizes of 2 and 

 256 words respectively. The order of reading the different size vocabu- 

 laries was randomized. 



Fig. 2 shows that reading rate is essentially independent of vocabulary 

 sizes from 4- to 256 words when familiarity and word length are kept fairly 

 constant. The reading rates for the three readers for the 256-word vo- 

 cabulary are 3.8, 3.7 and 3.0 words/sec, giving information rates of 30, 

 30 and 24 bits/sec respectively. 



The word rate for a 2-word vocabulary is systematically a little greater 

 than for larger vocabularies. This effect, which is statistically significant, 

 is best seen in Fig. 2 in the average curve (dashed). The writers feel on 

 the basis of subjective impressions that this may result from a tendencj'' 

 to group words in pairs in recognizing and speaking them. Among 2 

 words there are only 4 ordered pairs. It is apparent from the data that 

 no such effect is noted among the 16 ordered pairs occurring with the 

 4-word vocabulary. 



The last point on the curves in Fig. 2 illustrates the importance of 

 familiarity and word length. When words are taken at random from a 

 5,000-word dictionary (12.3 bits/ word), the reading rates drop to 2.8, 

 2.7 and 2.1 words/sec, yielding information rates of 34, 33 and 26 bits/sec 

 respectively, which are very close to the rates 30, 30, 24 for the 256-word 

 vocabulary. 



However, these dictionary lists involve some unfamiliar words and 

 average 2.2 syllables/ word. 



* When the light is very dim, the reading rate is slowed, and is faster for small 

 vocabularies than for large vocabularies. Reading tests were done at normal light 

 levels, which are very much brighter than those at which a slowing due to inade- 

 quate illumination is observed. 



