Set the trial rules before the first new meal
A diet trial only tells your vet something if the new food is the one thing that changed. Agree the rules with the clinic first: which food, how much each day, how the changeover from the old food is staged, and what counts as breaking the trial. Treats, dental chews, flavoured products, scavenged scraps, and another pet's leftovers can all quietly feed the old ingredients back in, so decide what replaces treat moments, perhaps a few pieces of the trial food itself, and tell everyone in the house. Write those rules at the top of your log, because six weeks in, nobody remembers exactly what was agreed on the first day, and the vet will ask.
Log the same fields every single week
Consistency is what makes weeks comparable. For each week record the exact food and daily amount actually eaten, not just offered, plus appetite and any meals refused. Score the signs the trial is targeting in plain, repeatable terms: stool firmness and frequency, vomiting, wind, itching or paw licking, ear redness or head shaking, and coat condition, noting better, worse, or unchanged beside each. Weigh your dog at the same time on the same day each week and write the figure down rather than rounding it. Date every entry as you make it, ideally each evening, because a week reconstructed on Sunday night smooths out the bad day that may be the most informative one.
Record the breaches honestly, not hopefully
Almost every home trial has slip-ups: a visitor hands over a biscuit, a child drops a sandwich, the dog raids the cat's bowl. Write each breach in the log with the date and what was eaten, instead of quietly writing the week off or hiding it. A vet judging fairly needs to know whether a flare in signs followed a breach by a day or two, which can actually strengthen the trial's conclusion, rather than suggesting the new food failed. Note illnesses, stress, boarding stays, and anything given for another reason in the same week, for the same reason: patterns only mean something when the exceptions are visible next to them.
Present the log so the recheck takes minutes
Before the appointment, read through the weeks and add a short closing note: when signs first shifted, which weeks were clean, and what you are unsure about. Bring the complete dated log, the food packaging details or a photo of its label, and your dog's current weight trend, so the vet can compare the trial against the signs recorded at the first visit. If the clinic keeps the log, keep your own copy with his health records. Photograph the food packaging and the measuring cup you actually use once at the start, so an amount written as one cup can be checked against the real cup weeks later, and so a quietly creeping portion gets caught. Storing the finished trial in KinsLe's Pet Passport keeps it in the same portable packet as his vaccination and health records, so a different vet can judge the change fairly too.