Factors that affect the capacity of STM

Reading out loud - Digit span increases if p's read the digits aloud instead of reading them subvocally. Baddeley (1999) suggested this because  the digits are also then stored briefly in the echoic store.

Individual difference - There is some evidence that individual differences affect STM capacity. For example, people who are highly anxious appear to have shorter spans (MacLeod and Donnellan, 1993)

Pronunciation time - A number of studies have shown that the capacity of STM is determined by time constraints rather than structural limitations. Naveh-Benjamin and Ayres (1986) compared memory spans for speakers of English with speakers of other languages. They found a direct relationship between span and pronunciation time. For example, Arabic speakers have a shorter digit span because they have longer words (takes longer to pronounce words compared to English speakers) 

Influence of LTM - Cowan (2000) believes that Miller might have overestimated the number of chunks that can be held in STM. He thinks that performance on span tasks is often affected by rehearsal and long term memory does not reflect the capacity of 'pure' STM and estimated that the capacity of STM is actually four chunks when such factors are controlled. Bower and Winzenz (1969) found that digit strings that were repeated within a series of immediate memory span trials become easier for p's to recall. This suggests that the strings have been gradually rehearsed and stored in LTM, thus temporarily increasing STM capacity.