Table: salyut 7
Question: Did the average duration of the expeditions on Salyut 7 decrease or increase over time? 
Step 1 (Operation, Indexing): duration (days)
Step 2 (Reasoning, Temporal Reasoning): launch date, over time
Step 3 (Operation, Indexing): launch date
Step 4 (Reasoning, Other Commonsense Reasoning): decrease or increase over time
Answer: decrease