Task switching is expensive (you've summoned the actors required for your current task onto your mental stage, and you have limited room on stage, so to handle a task switch you need to dismiss enough actors to make room for the new task, summon the actors required for the new task, then dismiss them and summon the others to return to your original task; and the break in your flow may be an excuse to pause or switch to easier tasks, and etc.).
Interruptions impose task switching on the person you interrupt.
But, interrupting someone else can also save you lots of time (for example, answering a question you would otherwise have to research, or making a decision you would otherwise have to wait for).
Getting the right balance between your time saved interrupting other people and their time lost to task switching is… an opportunity.


Interrupt levels
Different ways of contacting people have different inherent "interrupt levels", and agreed organisational conversational conventions can further tune these interrupt levels. Consider this approximately ordered list of interruption methods:

1. Calling out across a room (consider collateral damage to everyone else interrupted; consider just not doing this unless you want to interrupt everyone ("FIRE!"));
2. Physical approach & voice conversation;
3. Voice call (phone, Skype, etc.; convention says if you're busy you can not answer your phone or reject the call);
4. SMS to cell phone (I suggest holding SMS as higher than other Instant Messaging (IM) tools, because having more interrupt levels is good; convention says SMS will get to the target even if they're away from their workstation, but that they can screen or ignore SMS if they're busy);
5. Instant Message (Slack, Skype, etc.; convention says it's okay to screen IM and decide whether to answer it based on the subject and your task depth);
6. Email (convention says you should check your email at least daily, but not more often than that if you're deep in something).

Consider these interrupt levels & your control over them:

1. Calling out - nearly unignorable;
2. Physical approach & voice conversation - nearly unignorable;
3. Voice call (phone, Skype, etc.) - you can turn your phone off, but you have to answer it to work out what the subject of the call is;
4. SMS to cell phone - you can turn your phone off or screen the message;
5. Instant Message - you can silence these, or easily screen them (scanning an IM and deciding whether to respond doesn't take much attention compared to engaging with another human in conversation);
6. Email - you can decide when to check this (if you have an email notifier, uninstall it now).


Task depth
Different tasks require different depth of concentration, from tasks like challenging programming or mathematics with a very high cost of interruption, to processing your email inbox (not drafting that critical and complicated email reply) or scanning a document for grammatical errors with a very low cost of interruption.


Fitting all of this together
Interrupt other people at an appropriate interrupt level. If their answer isn't blocking you, use email. If it's critical, use a voice channel. If it's important, but you don't know whether it's as important as what they're working on and you don't know their task depth, use a clearly worded and terse IM so that they can screen your interruption and decide whether to break their flow.
Update your statuses based on your task depth. When working on a deep task turn off your IM. When working on a critical deep task, turn off your phone. Trust that others familiar with these conventions will understand that you're not ignoring their IM because you think that they're a bad person.
When someone interrupts you at an inappropriate interrupt level (eg. IMs you with something that's not blocking them (so they should have used email)), politely & with friendly demeanor suggest the interrupt level you think would have been appropriate.
When someone corrects you for using the wrong interrupt level, thank them for helping you to grow, and increase your affection for someone so bold and helpful.

When in the same room as others, consider other symbols of task depth - someone wearing headphones may be (is probably) signaling that they don't want to be interrupted. Someone with a red card attached to their headphones on which is written "Trying to concentrate" is almost certainly signaling that they really really don't want to be interrupted.
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