Waivers do not necessarily insulate liability if there is negligence deemed in many states as a lot of folks teaching in the equestrian community have found out. The laws for negligence vary by state, too, so Even with a waiver, insurance is still required because even if someone is ultimately not held accountable, there is cost to defend. it's essential to get legal advice from an attorney in that state to determine effectiveness, etc.
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The most expensive tool is the one you buy "cheaply" and often...