3.1. Data and Methodology
The hourly closing prices of the top ten cryptocurrencies by market capitalization—Bitcoin, Cardano, Chain-link, Dogecoin, Ethereum, Litecoin, Ripple, Tron, Tether and USD coin—are used to conduct this study. The data used for the study ranges from January 1st 2020 to 30th November 2023. All the data used for the analysis were downloaded from CoinMarketCap (
https://coinmarketcap.com/coins/). The reason for choosing the period is to incorporate the effects post COVID-19 period.
Market capitalization serves as a vital metric for assessing the value and size of a cryptocurrency (
Liu et al., 2022). Unlike individual cryptocurrency prices, which may not reflect the total value accurately, market capitalization provides a more comprehensive measure, allowing for comparisons with other cryptocurrencies. This concept mirrors the stock market, where market capitalization is calculated by multiplying the current stock price by the total number of shares outstanding. In the cryptocurrency realm, market capitalization is determined by multiplying the circulating supply (the number of coins available to the public) by the current price. Due to its significance, market capitalization is often a primary criterion for selecting cryptocurrencies. However, it should also be considered independently during portfolio optimization to ensure a well-rounded approach for practitioners.
The empirical analysis employs hourly returns due to their stronger kurtosis and their practical application in short-term strategic asset allocation.
Table 1 presents the main summary statistics for the cryptocurrency hourly return dataset, revealing well-known stylized facts such as negative skewness and positive excess kurtosis typically observed in financial assets.
Nine out of ten hourly average returns from all cryptocurrencies display positive values, indicating that all currencies, except USD coin, gained value over the analyzed period. However, these averages hover close to 0%, with the lowest positive return recorded by Tether and the only negative return observed with USD coin. Ripple boasts the highest hourly median return at 0.015%, alongside the largest range of 45.65%.
The median and mean values for the sample analyzed are nearly identical, differing by less than 0.02%. Skewness, a measure of data distribution symmetry, varies among the cryptocurrencies, with Bitcoin, Tron, and USD coin exhibiting near symmetry, while Cardano, Litecoin, and Ripple display high skewness toward the lower tail.
Volatility, assessed through standard deviation and coefficient of variation, remains high across all cryptocurrencies, with coefficient of variation absolute values ranging from 114.18 (Bitcoin) to 23,808.26 (Tether). None of the currencies exhibit normality at a 99% confidence level according to the Jarque-Bera test, as indicated by consistently low p-values (< 0.01). Additionally, all cryptocurrencies demonstrate high kurtosis, with values ranging from 18.29 (Ethereum) to 2185.35 (USD coin).
The combination of high volatility, skewness, and kurtosis in the majority of currencies underscores the need for a methodology allowing investors to understand currency behavior and formulate improved portfolios that actively perform better by controlling factors such as kurtosis.
We start by calculating the returns through the log difference of consecutive hourly closing prices of cryptocurrencies, following the method described by
Akyildirim et al. (
2021). This calculation follows the equation:
In this equation, Rₙ denotes returns on an nth hour in percentage; 𝐶𝑃ₙ denotes closing price on an nth hour; 𝐶𝑃₍ₙ₋₁₎ denotes closing price on the previous trading hour ; and In is a natural log.
After calculating the logarithmic returns, we construct the correlation matrix for its crucial role in portfolio management. It helps us understand the relationships between different assets within a portfolio, crucial for diversification and risk management (
Sukumaran et al., 2015). Accurate estimation of correlations is essential for constructing well-diversified portfolios (
Sukumaran et al., 2015) and controlling for the properties of the market factor included in a correlation matrix of stocks, vital for effective portfolio management (
Eom et al., 2015).
Secondly, the correlation matrix filters out random and systemic co-movements in constructing portfolios, improving risk profiles (
Zema et al., 2021). It enables capturing conditional volatility and correlation for forecasting future variance-covariance matrices between assets for optimal portfolio construction (
Razak, 2023). The correlation matrix is a fundamental component in portfolio optimization models, and its accurate estimation is crucial for their effectiveness (
Tola et al., 2008).
The following six steps generate the correlation matrix:
Step 1. Calculating Variance: The variance, a measure of dispersion, is computed using the formula:
where x represents each variable in the database and n is the sample size.
Step 2. Computing Covariance: Covariance, a measure of dispersion between two variables, is determined by:
where
denote pairs of variables, and
is the sample size.
Step 3. Generating Variance-Covariance Matrix (mxm): The variance-covariance matrix is a square matrix of size (mxm) obtained by mapping the covariances of each pair of variables in the database. Variances are placed on the diagonal:
where
is the number of variables.
Step 4. Calculating Correlation: Correlation between two variables is determined by dividing their covariance by the product of their standard deviations:
Where
: Standard deviation of the variable,
Step 5. Generating Correlation Matrix (mxm): The correlation matrix is a square matrix of size (mxm) that maps the correlation of each pair of variables. It contains a diagonal of 1:
Step 6. Calculating Portfolio Variance: Portfolio variance, a measure of returns dispersion, is computed by multiplying the transposed vector of weights by the variance-covariance matrix, then by the vector of weights:
where
is the vector of weights,
is the transposed vector of weights,
: is the variance-covariance matrix and
number of assets in the portfolio.
Figure 1 depicts the correlation matrix, showcasing that a few cryptocurrencies in the study exhibit notable correlation levels. However, literature suggests that these high correlations may not significantly impact portfolio optimization.
Antonakakis et al. (
2019) found substantial correlation among Bitcoin, Ripple, and Litecoin, yet these assets remain relatively isolated from traditional financial instruments, suggesting their potential for diversifying portfolios and implying that high correlations among cryptocurrencies may not hinder diversification efforts. Similarly,
Tzouvanas et al. (
2020) identified cryptocurrencies as promising assets for a well-diversified portfolio, particularly for short-term investments. This underscores that, despite pronounced correlations among cryptocurrencies, they may not undermine the diversification benefits.
The study maintains its validity by including high-risk portfolios consisting solely of risky assets, aimed at maximizing returns through diverse strategies. However, mitigating risk by incorporating assets with negative correlations poses a challenge with cryptocurrencies, especially considering their tendency to mirror the price and return behavior of Bitcoin (
Ji et al., 2019). Consequently, crafting a profitable cryptocurrency portfolio requires optimization. This process may pursue objectives such as maximizing the return-to-risk ratio, achieving maximum returns, or minimizing risk exposure.
After examining the correlation matrix, the next step is to construct a naive portfolio. In a naive portfolio, we allocate a fraction of 1/N of wealth to each of the N assets available for investment, following the naive rule. This approach commonly recommends the '1/N' allocation as a reference portfolio (
DeMiguel et al., 2011). In this study, we assigned 10% to each of the 10 assets in the portfolio. We conducted empirical investigations employing two distinct portfolios: one utilizing the Sharpe-maximization methodology, and the other employing Kurtosis minimization methodology.
Initially, for a T-period-long dataset of returns, we estimated the input parameters necessary for computing optimal portfolio weights according to the two strategies. Subsequent periods, expressed in weeks, were utilized to assess the portfolio performance over the following W periods, with W dictated by the applied rebalancing frequency. Throughout these W periods, we implemented a buy-and-hold approach, more pragmatic for real-world investors than a constant mixed approach. The process iterated by shifting the estimation window forward by W periods and recalculating the optimal portfolio weights. This iterative approach allowed for accumulating returns for subsequent W periods while discarding an equivalent number of earliest returns. This process repeated until reaching the end of the dataset, ensuring comprehensive evaluation and optimization of the portfolios.
3.3. Kurtosis Minimization Methodology Portfolio
The Kurtosis methodology involves utilizing a descriptive statistic known as kurtosis, which measures how data disperse between a distribution's center and tails. Larger values of kurtosis suggest that a data distribution may have "heavy" tails, meaning they are thickly concentrated with observations or have long tails with extreme observations (
Green, Manski, Hansen, & Broatch, 2023). This measurement is crucial for assessing dispersion or risk in financial assets.
In this methodology, the portfolio undergoes a process of optimization. This optimization entails searching for the lowest dispersion for each rebalancing period by selecting weights of the cryptocurrencies that minimize their kurtosis.
where,
: kurtosis of the variable
A normally distributed sample will have a kurtosis close to 3.
In the Pearson's kurtosis model, the kurtosis minimization methodology aims to regulate the kurtosis of the portfolio's assets by assigning lower weights to assets with higher kurtosis and vice versa. This calculation is performed during each rebalancing period to ensure the lowest possible kurtosis is achieved whenever currencies are traded.
Similar to the Sharpe maximization methodology, the same considerations and calculations were applied in this methodology. Rolling averages were utilized to smooth the highly volatile database of prices and returns, with the smoothing period determined by the strategy's timeframe. These rolled averages were then used to determine the portfolio weights that would result in the lowest possible kurtosis in each period. Subsequently, these weights were utilized in the subsequent holding period.
The formula for calculating the Kurtosis Minimization Portfolio Return is as follows:
represents the weight of asset n calculated with Kurtosis minimization in period t.
denotes the return of asset n in period t+1.
where,
: Sum of inverses of the kurtosis of all the variables in a portfolio.
The formula 11 ensures that the asset with the highest kurtosis receives the least weight of investment, while assets with lower kurtosis are assigned higher weights, thereby optimizing the portfolio for kurtosis minimization.