にわたって: the time or space an activity spans
Describe an activity extending over a period or area. Avoid overreading frequency or uninterrupted continuity, and learn noun-modifying にわたる.
How long did it span?
三日間にわたって、会議が開かれました。 means that a meeting was held over three days. It emphasizes that span, not necessarily 72 uninterrupted hours without sleep or breaks.
十年にわたって、この問題を研究しています。 says that the speaker has been researching this issue over ten years. ています presents research continuing to the present rather than reporting it as finished.
A broad area or multiple fields
広い地域にわたって、停電が起きました。 reports power outages across a broad area. The expression applies to space as well as time; it does not necessarily mean every building in the entire region lost power.
多くの分野にわたって、調査を行いました。 means that research covered many fields. It does not specify identical sample sizes or matching details in each field.
Modifying a noun and covering a whole period
三日間にわたる会議が終わりました。 uses にわたる to modify 会議. Distinguish the form before a noun from にわたって connecting the scope to an action.
一年を通じて花が咲きます。 suits a description of flowers available to see throughout the year. 一年にわたって調査しました。 gives the span of an investigation, so consider whether the focus is distribution through the whole period or the extent of an activity.
Quick check: Must 一週間にわたる研修 run continuously for a week with no breaks?
Show the answer and why
No. It means training spanning a week. A separate schedule is needed to know the daily hours and breaks.