Short and long-term memory have different capacities. Long term memories are thought to have an unlimited capacity whereas short term memories have a limited capacity of seven plus or minus two items according to Miller (956). It is believed that the number of items the short-term memory can hold can be increased via chunking. Vaughan and Green (984) studied pigeons to investigate the capacity of long-term memory. The pigeons were studied and appeared to have large long term memory capacity which is barely subject to decay. This study however is an animal study therefore is not ethical as animals cannot consent and cannot be generalised to humans, as a pigeon does not have the same brain as a human. However, this study is scientific and has contributed to our knowledge of memory capacity. A recent study using human participants was conducted by Brady et al. (2008) Participants were given five and a half hours to view 2,500 objects. They were then shown pairs of images and asked to state which they had previously seen. Results showed that participants had remembered thousands of images indicating that the capacity of long-term memory is very high. This study is ethical because participants all gave informed consent before participating . However, an issue with the study is that it does not reflect a real-life situation as it is not an everyday task to view 2,500 objects. Therefore, when generalising to everyday situations the study needs to be viewed with caution. Through these studies we can distinguish the difference between the capacity of both long- and short-term memory.
A difference between long and short-term memory is that both have different time frames in which memories are held. Long term memories are thought to be held for up to a lifetime whereas short term memories are thought to be held for around 8 seconds according to Peterson and Peterson (959). Within Peterson and Petersons (959) study 24
psychology students were given the task to recall three letter trigrams after different intervals of time. Students were told to count backwards in threes until they were asked to recall preventing any rehearsal of the trigrams. After three seconds around 80% of the recall was accurate however after eighteen seconds it had decreased to 0%. They concluded that short term memory has a limited duration of around eighteen seconds. This study uses artificial memories therefore lacks ecological validity and the results cannot be applied to everyday memory. However due to the highly controlled laboratories the results have been easy to replicate for example by Murdock (96). This therefore increases the reliability of Petersons and Petersons findings as the results are consistent. Furthermore, Bahrick et al. (975) investigated long term memory using 392 students. Participants were given photographs from their high school yearbook along with a selection of names and were asked to match the name with the photograph. It was found that after 4 years 90% of participants were able to match the names to the faces and after 47 years after graduation 60% of participants could correctly match the name to the photo. This led to Bahrick et al. (975) concluding that people can remember information such as names and faces for almost a lifetime. This provides evidence and support for the idea that long term memory has a duration of up to a lifetime. However, the sample Bahrick used was university graduates therefore this cannot be generalised to the general population as it lacks population validity. Although the sample lacks population validity the study has high levels of ecological validity, as it uses real life memories and does not rely on artificial memories therefore can be used to look at everyday memory. These studies therefore show how short- and long-term memories have different durations in which the memories are held.