Try . When you saw the test item, how did you decide whether it had been in the list you memorized? You probably answered quickly and automatically, without having to recite the list first.
You may not be aware of scanning a list in short-term memory, but that may be what happens, quickly and unconsciously. Saul Sternberg (1966) realized that he could use reaction times to determine how we scan short-term memory. There are three main possibilities: (1) You could examine the entire list at once and answer immediately. This is a parallel search. In that case, it should take about the same amount of time to answer no matter how long the list is (within reason) or whether the test item is present or absent. (2) You might actually go through the list items one-by-one, checking whether each matches the test item, and stop when you find the item or reach the end of the list. This is a serial self-terminating search. In this case, it will take longer to answer as list length increases. Furthermore, it will take about twice as long to answer when the test item is absent (because you must scan the whole list) than when it is present (because you stop when it is found). (3) You might even do a one-by-one serial search but go all the way to the end of the list even if you find the test item part way through your search. This is a serial exhaustive search. In that case, the time it takes to answer will depend only on the number of items in the list and will be the same whether the test item is present or absent.
Do (72 trials, about 15 minutes). Your reaction times will be measured, so it is important to respond quickly (use the arrow keys on the keyboard as instructed). However, your results will not be meaningful if you make many errors, so do not respond until you are sure of your answer. This was experiment #1 of Sternberg (1966). What type of search do your data suggest?
Your data may seem scattered, but the trends should show something surprising (see , for sample results). Your fast unconscious search was serial and exhaustive, apparently the least efficient of the three possibilities. Although it is clear that we use a serial exhaustive search in such tasks (a result that has been confirmed in other tests), it seems odd that the search does not end when the test item is found. One possibility is that interrupting the search to answer actually takes more time than finishing the search and then reporting the answer.
Fifty years after the initial publication, alternate interpretations of the experiment have been proposed. Sternberg (2016) reviews the evidence for and against, and concludes that serial exhaustive search remains the best explanation for memory scanning.