The Kibble Buffet Experiment

Cat IntroMy three indoor cats are all seniors now, so I’m even more concerned that they have a healthy diet. I feed them both wet and dry food. They share most of two cans of wet food per day and have dry food, kibble, available all the time. Kibble is like snack time. Fortunately, they are more disciplined than I am; they are all of normal weight despite having access to all that food.

With their advanced age, I was concerned that the kibble be easy on their digestive systems. I had been feeding them Iams for the past six years since I rescued them and they seemed content with it. Nevertheless, I went to Chewy to see if they had any alternatives for senior cats with sensitive stomachs. To say the least, I was quite surprised that Chewy had so many brands catering to what I thought was a limited feline demographic. So much for an exhaustive experiment I envisioned, this was going to be a pilot test limited to just a few brands.

If this were a critical scientific experiment that would face peer review and replication, there would be a lot to consider in planning. But this is just a small, personal experiment between just me and my cats. We’ll all be satisfied with the results whatever they may be. So, with that said, here’s the outline of the experiment.


The population for the experiment is small; it’s just my three indoor cats – Critter. Poofy, and Magic. Critter (AKA Ritter Critter) was rescued by my daughter from outside the Ritter building at Temple University when she was a kitten in 2006. Poofy (AKA Poofygraynod) was rescued from my back yard in 2013 when (the vet thought) she was about 6. Magic (AKA Black Magic) was also rescued (with Poofy) from my back yard in 2013 when (the vet thought) she was about 2. Critter and Magic were born in the wild; Poofy might have been a stray. At the time of the experiment, Critter and Poofy were about 14 and Magic was about 10. Poofy weighs about 13 pounds, Critter weighs about 11 pounds, and Magic weighs about 8 pounds, Poofy eats mostly kibble but also some wet food. Critter eats mostly wet food (ocean whitefish is her favorite) but also some kibble. Magic eats both kibble and wet food equally, in good feral fashion.

Cat population

Because the population consists of only three individuals, and it’s their composite response that is of interest, the experiment actually involves measuring the cats’ preferences repeatedly over the course of the experiment. This is a census (rather than a survey) of the population. The sampling design is systematic, one set of measurements of kibble eaten by brand for the duration of the experiment. This is called a repeated measures design.


The phenomenon being evaluated is preference for selected brands of kibble. Each cat may have a different preference, even changing day-by-day, but only the composite preference is important because I purchase the kibble in the aggregate. Preference is measured by the amount of each brand of kibble consumed in a 24-hour period.

Research Questions

I had four research questions I wanted to answer.

  1. What kinds of kibble do my indoor cats like to eat?
  • I hypothesized that they might prefer Iams since that is the brand they had been eating for the prior seven years.
  • I hypothesized that they might like seafood best because this preference is often depicted in cat-related cliche.
  • Protein, Fat, Fiber. I hypothesized that they might prefer the highest protein content.
  • I hypothesized that they might like kibble that was smaller and more rounded so that it was easier to swallow.
  1. How much do they eat in a day? I hypothesized that they would eat less that three cups of food per day based on prior feeding patterns. I provided a total of about twice that amount during the experiment, one cup of kibble for each of the six brands each day.
  2. Do they prefer variety or will they eat the same kibble consistently? I hypothesized that they would eat a variety of the brands because I like variety in MY diet. My vet disagreed. He thought they would eat what they were most familiar with.
  3. Will a different kibble reduce their barfing? I hypothesized that there would be no difference because they were already eating Iams kibble for cats with sensitive stomachs.

Kibble Brands

I was surprised by how many different brands there are of dry food for senior cats with sensitive stomachs available from just one vendor (Chewy). I decided to test just six because of cost and test logistics. They were:

Iams – because that’s the brand I had been feeding the cats for the last seven years; it was my “control group” brand. It is the least expensive of the brands and consists primarily of chicken and turkey, corn, rice, and oats.

Ingredients Iams

Purina – because I wanted a well-known national brand available in supermarkets. I selected two Purina products, Focus and True Nature, to see if there was a difference between kibble formulations from the same company. They have the largest kibble, high protein, high calories, and tend to be more expensive. Focus is chicken and turkey flavored and contains rice, oats, and barley. True Nature is salmon and chicken flavored, and is grain free.

Ingredients Focus

Ingredients True Nature

Hills Science Diet – because I wanted a well-known, highly-rated, vet-recommended brand sold mostly in pet stores. It is high density and high calorie but lower protein than the other brands. It is chicken flavored and contains corn, soy, and oats.

Ingredients Hills

Halo – Perhaps the first “holistic” cat food, having been introduced in the 1980s, it purports to be ultra-digestible because of its use of fresh meats, vitamins, probiotics, and other healthy ingredients. It is seafood flavored and contains oats, soy, and barley.

Ingredients Halo

I and Love and You – A newer formulation of holistic ingredient, it is grain-free, includes prebiotics and probiotics for healthy digestion, and has the longest ingredient list by far. It contains seafood and chicken/turkey.

Ingredients I Love You

Experimental Procedure

The experimental set up consisted of six paper bowls, one for each brand of kibble. The positions of the bowls were randomized so that the cats wouldn’t associate a certain kibble brand with a position. Every 24 hours, at 8 PM, the bowls were filled with one cup of kibble and weighed. (They still got their can of wet food at 5 AM when they wake me up.) The cats were then allowed to eat the kibble as they wanted for 24 hours. It was clear from the beginning of the experiment that the cats did prefer certain brands, though they would try others.

Bowls 1-IMG_5859

At the end of 24 hours, the bowls were reweighed. Remaining kibble was transferred to a bucket to be fed to my three outdoor feral cats, who will eat anything. The bowls were then filled and weighed again, and placed in their new randomized position.

Data Recorded each day included:

  • Day of experiment and time
  • Kibble brand
  • Bowl position
  • Weight of kibble not eaten
  • Weight of kibble provided for the next day

An informal inspection of the house was also conducted to identify any barfs that may have occurred.

Cats Three-IMG_5863

Planned Analysis

The dependent variable for the analysis was the brand of kibble. The independent variable for the analysis was the weight of kibble eaten, calculated as:

Weight of kibble eaten = Weight of kibble provided – Weight of kibble left over

The position of the bowls was a blocking factor used to control extraneous variance. The day-of-the-experiment was a repeated-measures factor. This design is a two-way repeated-measures Analysis of Variance (ANOVA).

Prior to conducting any statistical testing, an exploratory analysis was planned involving calculating descriptive statistics and constructing graphs.

Depending on the results of the exploratory analysis, global ANOVA tests were planned for detecting differences between the brands, with the effects of bowl position and day of the experiment held constant. A priori tests were also planned to detect any differences between individual brands and the control brand, Iams.

Cats Two-IMG_5873


Though not what I expected, it was obvious after a couple of days that the cats had a clear preference for Hills Science Diet. Consequently, I ended the experiment after two weeks.

Bowls 3-IMG_5868

Descriptive Summary

The following table summarizes the amount of each brand that the cats ate over the two-week experiment.

Table results

Statistical Testing

While the design of the experiment is technically a two-way repeated-measures Analysis of Variance (ANOVA), the large differences in brands and the lack of differences in bowl position and day of the experiment made calculating the model unnecessary. This solved the problem of my not having appropriate software to conduct that part of the analysis. Instead, the following sections describe two-way ANOVA results for the brand versus bowl position and the brand versus day of the experiment models. Statistical comparisons of the amounts of each brand eaten are also summarized, with emphasis on differences between each brand and the “control” brand, Iams.

Cats Two-IMG_5861

Table Kibble by Day

Bar chart day

The global ANOVA test for the brands was significant when the effects of the day of the experiment was controlled for. The day of the experiment had no impact on the amount of kibble eaten. No surprise there.

ANOVA Brands Days

Cats Two-IMG_5871

Table Kibble by Position

Bar chart bowl position

The global ANOVA test for the brands was significant when the effects of bowl position was controlled for. Bowl position had no impact on the amount of kibble eaten. Again, that’s not a big surprise although you can never be sure of a hypothesis when you’re working with cats.

ANOVA Brands Position

Cats One-IMG_5869

The following table summarizes the statistical tests between the brands. The important tests are the comparisons between each brand and the control brand, Iams, highlighted in yellow. The only significant tests were the comparisons between Hills Science Diet versus Iams and Purina True Nature versus Iams. This means that my cats like Hills and True Nature a lot more than what I’ve been feeding them for the last few years. Time to switch brands. I could have done worse had I fed them one of the other brands instead of Iams, but not significantly so.

ANOVA Post hoc

Cats Two-IMG_5870


First, things don’t always go the way you think they will. This is true in any experiment … and life in general. There was really no need to conduct the sophisticated ANOVA that I had planned, so I didn’t bother. Oh well, next time.

Second, my three cats eat about 135 grams (4.8 oz) of kibble in a day. Now I can use the automatic reorder feature on Chewy and save a few dollars.

Third, You know how you tend to eat a lot more after you come home with new groceries? Cats do it too. They ate a lot more on the first day of the experiment when they had five new brands of kibble to taste.

Fourth, I randomized bowl position as a way of controlling extraneous variation for the ANOVA. It seems that the middle positions had more kibble eaten per bowl than the outer positions. I have no explanation for this pattern and the cats aren’t talking.

Fifth, I can’t say that it reduced barfing because I had no baseline. My daughter, who doesn’t like cats, led me to believe that they barf about every twenty minutes. Still, there were only three barfs during the two-week experiment, which I considered not to be so bad.

Sixth, my cats clearly prefer Hill’s Science Diet as their kibble of choice. They don’t seem to want a variety of brands. I don’t know why my cats preferred Hills. It doesn’t appear to be the flavor, texture, or protein content since other brands had different combinations of these factors. If you look at reviews of other brands of kibble, you’ll find people who swear that their cat(s) likes the-brand-that-they-buy best. They’re probably right. Every cat or population of cats may have different tastes. I, myself, like pineapple on my pizza.

Finally, my experiment wasn’t large or sophisticated enough to isolate and analyze hypotheses about ingredients. If I could figure out why cats prefer one brand of kibble over another, though, I could probably get a job with Purina.

Cats Two-IMG_5865

Further Research

It’s always a good practice to describe additional research that could be done to make the world a better place. Who knows if somebody with money might see it and fund your further research? In this case, further research might involve testing different brands, especially if the brands could be selected to explore a variety of flavors, kibble shapes, sizes, densities, and types and concentrations of protein. Finally, I would recommend using many, many more cats if you can. My daughter won’t let me have any more.

So, if you find yourself with some time on your hands, consider conducting your own experiment on your cats. You might be surprised at what you learn. It’ll be fun.

More cats

Read more about using statistics at the Stats with Cats blog. Join other fans at the Stats with Cats Facebook group and the Stats with Cats Facebook page. Order Stats with Cats: The Domesticated Guide to Statistics, Models, Graphs, and Other Breeds of Data analysis at, or other online booksellers.

About statswithcats

Charlie Kufs has been crunching numbers for over thirty years. He retired in 2019 and is currently working on Stats with Kittens, the prequel to Stats with Cats.
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2 Responses to The Kibble Buffet Experiment

  1. So glad to see a post again after a 16 month hiatus! You write with such clarity and translation from the technical to the practical implications. I share your “how to write data analytics report” posts with my introductory data analysis class before their final projects.

  2. abonifonte says:

    So glad to see a post after your 16 month hiatus! You write with such clarity and transition from the technical to the practical implications. I share your ‘how to write data analysis reports’ posts with my intro class before their final projects.

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