For more information, read an Overview of the Methods & Calculations.
The steps of the multivariate Markov Chain Monte Carlo (MCMC) method are:

Separating. Each HEI dietary constituent must be treated separately. Unlike other methods, dietary constituents are not summed and must be separated for statistical modeling.
For the HEI2015, the components and associated dietary constituents are noted below:
HEI2015 Components and Associated Dietary Constituents for MCMC MethodHEI Component MCMC Variables Type^{a} Units Dietary Constituents From FPED (or other foodbased database) Total Fruits Whole Fruit Episodic cup eq. Citrus, Melons, Berries + Other Intact Fruits Fruit Juice Episodic cup eq. Fruit Juice Whole Fruits Whole Fruit Episodic cup eq. Citrus, Melons, Berries + Other Intact Fruits Total Vegetables NonDark Green Vegetables Daily cup eq. Total Vegetables – Dark Green Vegetables^{b} Dark Green Vegetables Episodic cup eq. Dark Green Vegetables Legumes (Beans and Peas) Episodic cup eq. Legumes (Beans and Peas) Greens and Beans Dark Green Vegetables Episodic cup eq. Dark Green Vegetables Legumes (Beans and Peas) Episodic cup eq. Legumes (Beans and Peas) Whole Grains Whole Grains Episodic oz. eq. Whole Grains Refined Grains Refined Grains Daily oz. eq. Refined Grains Dairy Dairy Daily cup eq. Total Dairy Total Protein Foods Meat, Poultry and Eggs^{b} Daily oz. eq. Total Meat, Poultry (including organ meats and cured meats) + Eggs Seafood, Soy, and Nuts and Seeds Episodic oz. eq. Seafood (high in n3) + Seafood (low in n3) + Soy + Nuts and Seeds Legumes (Beans and Peas) Episodic oz. eq. Legumes (Beans and Peas) Seafood and Plant Proteins Seafood, Soy, and Nuts and Seeds Episodic oz. eq. Seafood (high in n3) + Seafood (low in n3) + Soy + Nuts and Seeds Legumes (Beans and Peas) Episodic oz. eq. Legumes (Beans and Peas) Added Sugars Added Sugars Daily tsp. eq.* Added Sugars From FNDDS (or other nutrient database) Fatty Acids Fatty Acids Daily g Total Monounsaturated Fatty Acids + Total Polyunsaturated Fatty Acids Saturated Fats Saturated Fats Daily g* Total Saturated Fatty Acids Sodium Sodium Daily mg Sodium  Energy Daily kcal Total Energy cup eq.=cup equivalents; oz. eq.=ounce equivalents; tsp. eq.= teaspoon equivalents; tsp. eq*.=added sugars are calculated in teaspoon equivalents but converted to energy in the scoring process; g=grams; g*= fatty acids are calculated in grams but converted to energy in the scoring process; mg=milligrams
^{a} General guidance on daily vs. episodic. Should be examined for each individual dataset.
^{b} Should be rounded to 2 decimal places and set to 0.00 if value is negative.

Modeling. Using a model that: a) transforms data to approximate normality, b) separates withinperson from betweenperson variability, c) models the probability of consumption separately from the consumption day amount for episodically consumed foods, d) allows for correlation among the probability of consumption, the consumptionday amount, and energy among all dietary constituents; usual intakes for each dietary constituent are predicted.

Deriving Sums. After usual intake is predicted the constituents are combined to create the variables corresponding to the HEI components.

Constructing Ratios. The appropriate ratios are constructed. Usually these are the ratios of the dietary constituents to 1000 kcal of energy, with the exception of fatty acids, which use the ratio of the sum of monounsaturated and polyunsaturated fatty acids to saturated fatty acids. (Also, note two components are expressed on a percent of calories basis. Therefore, grams of saturated fat should be multiplied by 9 to convert g to kcal, and added sugars should be multiplied by 16 to convert teaspoons to kcal, prior to dividing by total energy.) Step 5 and beyond are different depending on purpose; see two options below.
For Describing Dietary Intakes: Other methods are under development by NCI Biometry Research Group to examine the association between diet and another variable. For more information, see their resources on measurement error.

Estimating. Generate “pseudoindividuals” from the parameters estimated from the model fit in Step 1.

Scoring. For each of the pseudoindividuals, score the ratios according to the scoring standards for each component. The component scores are summed to calculate the total score.

Calculating Means. Compute means and percentiles for the population of pseudoindividuals.