Signaling devices could undermine the recall of topics unrelated to the sigaling devices. So testing on the overall recall would not be accurate. 

Why testing on overall recall is a problem 

Text with a simple text structure would be easy to recall, so the signaling devices would present little effects in improving the free recall. 

Simple text is already easy to recall 

1. Meyer’s experiments tested the overall recall 
2. Text is too short and has a relatively simple structure 

University of Michigan - Ann Arbor

One of the researchers Meyer has conducted several experiments on investigating whether headings would improve people recalling the information, and the results showed that headings would not improve people recalling information. 

Previous Experiments have failed in supporting the beneficial effects of headings on overall recall

Lorch, R. F., Lorch, E. P., &amp; Inman, W. E. (1993). Effects of signaling topic structure on text recall. Journal of Educational Psychology, 85(2), 281–290. https://doi.org/10.1037/0022-0663.85.2.281 

Effects of Signaling Topic Structure on Text Recall

Flaws of the experiments 

1. examine only the free recall for information directly related to the signaling devices
2. examine the free recall for information in the text content  

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