Retailer Y uses (for client operation purposes) a network of about 3000 devices. The whole network has 10 different models (from distinct manufacturers), aged from 0 years old (purchased in the current year) to 12 years old.
Each device may suffer from malfunctions for which immediate repair is requested. The total time of unavailability for each device is accounted at the end of month and we call it downtime.
The same device may have no malfunction during that month period or may have one or more unavailability periods during that month period.
Common sense says that older equipment should have higher downtimes.
(a) How can I check whether the Age variable has effective impact over the Downtime variable? Which statistical model should I use?
(b) How can I account for the fact that each type of equipment should behave differently to aging regarding downtime?