Style of writing.
The style of writing that is appropriate for a job advertisement depends on the nature of the position. Once you have caught your reader's attention you should proceed swiftly to giving information.
Job advertisements : whose responsibility?
If the post is new or has changes considerably, the line manager should be more involved in deciding the content, and more expertise in composing advertisements will be needed so someone with more experience in this area and well-developed writing skills should take on the task.
If the post is difficult to recruit for (maybe due to local skills shortage) you may consider using the specialized skills of a recruitment advertising agency. The additional cost of this would be a factor to weigh against the cost of unsuccessful recruitment attempts.
Targeted Recruitment.
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