The ability to give and receive honest, constructive feedback is one of the signs of a successful employer/employee relationship. Giving feedback is a crucial factor in managing employee performance, and just because your organization has a formal employee performance management system in place, it doesn't mean that informal discussions can't be had between official appraisals and meetings.
In fact, sometimes informal feedback can be more productive at nipping a specific performance issue in the bud before it escalates, because it gives the employee the chance to recognize and rectify whatever's causing their underperformance before the situation becomes formal.
See also, the four golden rules for delivering feedback to employees.
One recognized method of giving constructive feedback is the BOOST feedback model, a way that can encourage employees to rectify a performance issue or repeat positive behaviours.
Balanced - Feedback should always be balanced and constructive to achieve the best results, so even if the person giving feedback is bringing up an area for development, they should make sure that strengths are mentioned too. It also needs to be two-way, so an employee should be asked their opinion of their performance before any feedback is given. There may well be a mitigating factor that management aren't aware of.
Observed - It must be something that the person giving the feedback has observed themselves for the feedback to be valid. It's impossible to be constructive if someone's telling an employee that they've 'heard' that they were underperforming in some way. After all, what's been 'heard' may not even be true if the person giving feedback is relying on information from other people.
Objective - Feedback should always be factual and based on an employee's behaviours and actions, not the personal opinion of whoever's giving the feedback. It's sometimes difficult to do this, but if someone recognizes that they'd struggle to be objective giving feedback, they should have the ability to pass the task on to another appropriate person. Objective feedback should focus on the impact the underperformance is having, without personal feelings about the employee colouring the discussion.
Specific - Performance can't be addressed successfully without having specific examples, which lets the employee respond and address the issues. Feedback needs to be about specific examples that the employee can actually do something about, rather than something general along the lines of "You need to concentrate more". What do they need to concentrate on? What problems have occurred due to their lack of concentration?
Timely - It probably goes without saying, but issues need to be tackled straight away, as soon after the event as possible. This means that situations are fresh in both people's minds, meaning feedback is as accurate as possible and all of the details are easy to remember.
If you're looking at an issue of underperformance, the employee is likely to appreciate a less formal approach. Informal feedback using the BOOST model encourages performance improvement by allowing a two-way discussion. It's not just for underperformance either; using the BOOST model for positive feedback is also valuable in managing employee performance.
If you're looking for an online tool to help you organize performance appraisals and keep feedback flowing, check out Primalogik 360 - employee performance management software